Bumped Travis CI to Go version 1.12.3.
Bumped Travis CI to Go version 1.12.3.

This commit bumps the Go version used on Travis CI (for testing and
builds) to Go 1.12.3. It notably does not bump the AppVeyor Go version
to 1.12, since Go 1.12 isn't (easily) available there yet.

Since we need to maintain support for Go 1.11 for a while longer,
Mutagen's version requirements have been left at Go 1.11.

havoc-io/mutagen,havoc-io/mutagen