Optionally choose the go version from go.mod

[brackendawson]

closes #199

[brackendawson]

Despite my access on this repository, I'm not really comfortable merging new functionality here without Travis folks being on board too (and they seem to ignore this repository 😞).

Do you think a Travis Enterprise customer prodding them would help? I'll see what I can do.

[meatballhat]

Speaking as a former Travis folk, I'd say forking this repo to another org where it can be more easily maintained by non-Travis folk is A Great Idea and I'm happy to suggest a destination 😁

[brackendawson]

It turns out being a Travis enterprise customer doesn't help. Forking doesn't really help my use case because I want Travis to have this feature. I'm not opposed though.

[carmanchris31]

Are we any more confident merging this now? 🤞