script/build-docker fails for darwin/amd64

Docker builds fail for `go 1.7.5 darwin/amd64`, please advise

```sh
$ script/build-docker -os="darwin" -arch="amd64"
Sending build context to Docker daemon 35.95 MB
Step 1/6 : FROM golang:1.8.1
---> fcef75ee6be1
Step 2/6 : ENV USER root
---> Using cache
---> bd18eb02aa78
Step 3/6 : WORKDIR /go/src/github.com/cloudflare/cfssl
---> Using cache
---> 14f0919593fb
Step 4/6 : COPY . .
---> 4897e3e8666d
Removing intermediate container e254e4caff61
Step 5/6 : RUN go get github.com/mitchellh/gox
---> Running in b7a2f80ae70e
---> 93fb810e5640
Removing intermediate container b7a2f80ae70e
Step 6/6 : ENTRYPOINT gox
---> Running in 42d944745803
---> 53b28a0595d7
Removing intermediate container 42d944745803
Successfully built 53b28a0595d7
Number of parallel builds: 3

--> darwin/amd64: github.com/cloudflare/cfssl/cmd/cfssl-newkey
--> darwin/amd64: github.com/cloudflare/cfssl/cmd/multirootca
--> darwin/amd64: github.com/cloudflare/cfssl/cmd/cfssl-bundle
--> darwin/amd64: github.com/cloudflare/cfssl/cmd/cfssl
--> darwin/amd64: github.com/cloudflare/cfssl/cmd/cfssl-certinfo
--> darwin/amd64: github.com/cloudflare/cfssl/cmd/cfssljson
--> darwin/amd64: github.com/cloudflare/cfssl/cmd/cfssl-scan
--> darwin/amd64: github.com/cloudflare/cfssl/cmd/mkbundle

7 errors occurred:
--> darwin/amd64 error: exit status 2
Stderr: # github.com/cloudflare/cfssl/vendor/github.com/google/certificate-transparency-go/x509
vendor/github.com/google/certificate-transparency-go/x509/root_stub.go:9: (*Certificate).systemVerify redeclared in this block
previous declaration at vendor/github.com/google/certificate-transparency-go/x509/root_darwin.go:26
vendor/github.com/google/certificate-transparency-go/x509/root_stub.go:13: initSystemRoots redeclared in this block
previous declaration at vendor/github.com/google/certificate-transparency-go/x509/root.go:20

--> darwin/amd64 error: exit status 2
Stderr: # github.com/cloudflare/cfssl/vendor/github.com/google/certificate-transparency-go/x509
vendor/github.com/google/certificate-transparency-go/x509/root_stub.go:9: (*Certificate).systemVerify redeclared in this block
previous declaration at vendor/github.com/google/certificate-transparency-go/x509/root_darwin.go:26
vendor/github.com/google/certificate-transparency-go/x509/root_stub.go:13: initSystemRoots redeclared in this block
previous declaration at vendor/github.com/google/certificate-transparency-go/x509/root.go:20

--> darwin/amd64 error: exit status 2
Stderr: # github.com/cloudflare/cfssl/vendor/github.com/google/certificate-transparency-go/x509
vendor/github.com/google/certificate-transparency-go/x509/root_stub.go:9: (*Certificate).systemVerify redeclared in this block
previous declaration at vendor/github.com/google/certificate-transparency-go/x509/root_darwin.go:26
vendor/github.com/google/certificate-transparency-go/x509/root_stub.go:13: initSystemRoots redeclared in this block
previous declaration at vendor/github.com/google/certificate-transparency-go/x509/root.go:20

--> darwin/amd64 error: exit status 2
Stderr: # github.com/cloudflare/cfssl/vendor/github.com/google/certificate-transparency-go/x509
vendor/github.com/google/certificate-transparency-go/x509/root_stub.go:9: (*Certificate).systemVerify redeclared in this block
previous declaration at vendor/github.com/google/certificate-transparency-go/x509/root_darwin.go:26
vendor/github.com/google/certificate-transparency-go/x509/root_stub.go:13: initSystemRoots redeclared in this block
previous declaration at vendor/github.com/google/certificate-transparency-go/x509/root.go:20

--> darwin/amd64 error: exit status 2
Stderr: # github.com/cloudflare/cfssl/vendor/github.com/google/certificate-transparency-go/x509
vendor/github.com/google/certificate-transparency-go/x509/root_stub.go:9: (*Certificate).systemVerify redeclared in this block
previous declaration at vendor/github.com/google/certificate-transparency-go/x509/root_darwin.go:26
vendor/github.com/google/certificate-transparency-go/x509/root_stub.go:13: initSystemRoots redeclared in this block
previous declaration at vendor/github.com/google/certificate-transparency-go/x509/root.go:20

--> darwin/amd64 error: exit status 2
Stderr: # github.com/cloudflare/cfssl/vendor/github.com/google/certificate-transparency-go/x509
vendor/github.com/google/certificate-transparency-go/x509/root_stub.go:9: (*Certificate).systemVerify redeclared in this block
previous declaration at vendor/github.com/google/certificate-transparency-go/x509/root_darwin.go:26
vendor/github.com/google/certificate-transparency-go/x509/root_stub.go:13: initSystemRoots redeclared in this block
previous declaration at vendor/github.com/google/certificate-transparency-go/x509/root.go:20

--> darwin/amd64 error: exit status 2
Stderr: # github.com/cloudflare/cfssl/vendor/github.com/google/certificate-transparency-go/x509
vendor/github.com/google/certificate-transparency-go/x509/root_stub.go:9: (*Certificate).systemVerify redeclared in this block
previous declaration at vendor/github.com/google/certificate-transparency-go/x509/root_darwin.go:26
vendor/github.com/google/certificate-transparency-go/x509/root_stub.go:13: initSystemRoots redeclared in this block
previous declaration at vendor/github.com/google/certificate-transparency-go/x509/root.go:20
```

`go env` output:

```
GOARCH="amd64"
GOBIN=""
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/gosuri/code/go"
GORACE=""
GOROOT="/usr/local/Cellar/go/1.7.5/libexec"
GOTOOLDIR="/usr/local/Cellar/go/1.7.5/libexec/pkg/tool/darwin_amd64"
CC="clang"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/mh/3j05rkl55cj_4xrfdkpqxr9h0000gn/T/go-build707407111=/tmp/go-build -gno-record-gcc-switches -fno-common"
CXX="clang++"
CGO_ENABLED="1"
```

Also, binary exits with 137 status code when installed using go install `go install github.com/cloudflare/cfssl/cmd/cfssl`

```sh
$ cfssl
zsh: killed cfssl
$ echo $?
137
```