certificate transparency fails to build with -os="darwin"

Not sure if this is certificate transparency's issue or ours (I'm not familiar with gox):

```
$ go version
go version go1.7.1 linux/amd64

$ go env
GOARCH="amd64"
GOBIN=""
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/vagrant/src/go"
GORACE=""
GOROOT="/usr/local/src/go"
GOTOOLDIR="/usr/local/src/go/pkg/tool/linux_amd64"
CC="gcc"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build141194028=/tmp/go-build -gno-record-gcc-switches"
CXX="g++"
CGO_ENABLED="1"

$ script/build -os="darwin" -arch="amd64"
Number of parallel builds: 1

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

6 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:11
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:11
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:11
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:11
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:11
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:11
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
```

Tagging @jacobhaven as he's previously fixed a darwin issue with this dependency.

The other darwin architectures also fail with the same issue.