support for multiple versions of go

[sjkaliski]

Currently this project defaults to the latest stable version of Go, 1.11.x, via the golang:1.11 docker image.

A couple questions come up:

1. How do we handle Go updates in this package?
2. Do we need to provide backwards support for 1.9.x, 1.10.x?

For 1, seems like a good option is to update once the new minor version is out and stable & then publish a new release. Any new updates moving forward would only support the latest Go version.

For 2, not sure just yet. Thoughts?