language: go

go:
  - 1.4
  - 1.5
  - 1.6
  - tip

language: go

go:
  - 1.4
  - 1.5
  - 1.6
  - 1.7.x
  - master

Fix Travis build file and add latest go 1.7
Fix Travis build file and add latest go 1.7

This removes the tip spec to use master instead, because
tip is apparently not supported anymore, see https://github.com/travis-ci/gimme/issues/38
and https://docs.travis-ci.com/user/languages/go.
Also, using the `x` suffix is apparently necessary to use the latest version.
