Added TODO item for adding version constraints in Go 1.8.
Added TODO item for adding version constraints in Go 1.8.

We're only going to support the latest version of Go, and a build
constraint in the version file is the best way to do this. We require
version 1.7.3+ at the moment, but there's no way to express that, so
we'll just wait for Go 1.8 before we start adding hard requirements.

havoc-io/mutagen,havoc-io/mutagen