Simplify check for current stable version
Simplify check for current stable version

golang.org exposes an API just for this, which should always be up to date.

The current implementation is convoluted and error-prone (I am currently seeing it return Go 1.7.6 as stable), so this should be an improvement.
glevand/travis-ci--gimme,meatballhat/gimme,travis-ci/gimme,travis-ci/gimme,glevand/travis-ci--gimme