CI: Drop Go 1.6 on osx to speed things up
CI: Drop Go 1.6 on osx to speed things up

Travis' osx build backlog is painfully long, and we can assume that Go
1.6 succeedes on macOS if it does for Go 1.8 and Go 1.6 on Linux.

The most important thing is that it builds with the latest Go release on
macOS.

la5nta/pat,la5nta/pat,la5nta/pat,la5nta/pat