Bump go version for Travis
Bump go version for Travis

- Use "1.12.x" to float on the current available 1.12 patch version

[#169026398](https://www.pivotaltracker.com/story/show/169026398)

cloudfoundry-incubator/cli-plugin-repo,cloudfoundry-incubator/cli-plugin-repo,kkellner/cli-plugin-repo,kkellner/cli-plugin-repo,cloudfoundry-incubator/cli-plugin-repo,cloudfoundry-incubator/cli-plugin-repo,kkellner/cli-plugin-repo