add go modules

It's useful to add a go.mod file. The build in Travis is broken because not find some dependencies

And the tags are not semver and so the project cannot be imported into a go.mod. Creating a new tag v1.0.0 based on the current v1.0 will really help, thanks.

For help here is your go.mod file:

module github.com/bndr/gojenkins

require (
	github.com/stretchr/testify v1.3.0
	golang.org/x/net v0.0.0-20180901120000-a4630153038d
)

+1