CI builds are broken after go modules

The CI builds are broken after the migration to Go modules.

```console
$ travis_install_go_dependencies 1.12.x -v
Makefile detected

$ export SOURCE_DATE_EPOCH=$(git show -s --format=%ci ${TRAVIS_TAG:-${TRAVIS_COMMIT}})
The command "export SOURCE_DATE_EPOCH=$(git show -s --format=%ci ${TRAVIS_TAG:-${TRAVIS_COMMIT}})" exited with 0.

$ go get github.com/mitchellh/gox
go get: warning: modules disabled by GO111MODULE=auto in GOPATH/src;
ignoring go.mod;
see 'go help modules'
The command "go get github.com/mitchellh/gox" exited with 0.

$ GOFLAGS=-mod=vendor gox -output="{{.Dir}}-{{.OS}}-{{.Arch}}-${TRAVIS_TAG:-${TRAVIS_COMMIT}}" -os='darwin dragonfly freebsd linux netbsd openbsd solaris' -osarch='!dragonfly/386 !darwin/arm64 !darwin/arm !linux/mips !linux/mipsle' -gcflags="-trimpath=${GOPATH}" ./cmd/certmgr/...
Number of parallel builds: 1
The command "GOFLAGS=-mod=vendor gox -output="{{.Dir}}-{{.OS}}-{{.Arch}}-${TRAVIS_TAG:-${TRAVIS_COMMIT}}" -os='darwin dragonfly freebsd linux netbsd openbsd solaris' -osarch='!dragonfly/386 !darwin/arm64 !darwin/arm !linux/mips !linux/mipsle' -gcflags="-trimpath=${GOPATH}" ./cmd/certmgr/..." exited with 0.

$ for i in certmgr-*; do tar --mtime="${SOURCE_DATE_EPOCH}" --owner=0 --group=0 --numeric-owner -c $i | gzip -n - > $i.tar.gz; done
tar: certmgr-*: Cannot stat: No such file or directory
tar: Exiting with failure status due to previous errors
The command "for i in certmgr-*; do tar --mtime="${SOURCE_DATE_EPOCH}" --owner=0 --group=0 --numeric-owner -c $i | gzip -n - > $i.tar.gz; done" exited with 0.

$ shasum -a 512 certmgr-*.tar.gz | tee sha512sum.txt
627d38af59027d716d6c8f8be41c0d1b9855259f5887607ddfd7789593e4708c2a8e1f0565f0c3f4a59e558c404d03dbc427effce87bcf806172dd92bc4c862d certmgr-*.tar.gz
The command "shasum -a 512 certmgr-*.tar.gz | tee sha512sum.txt" exited with 0.
```