Updated travis build scripts (from rspec-dev): Pin to minor versions

Created by: pirj

Having patch versions pinned comes with a burden of sending such pull requests once in a while. We can depend on Travis to pick the latest patch version as RVM updates.

On the other hand, problems with the patch versions would be discovered later, during the next pull request. This, however, is unlikely to happen too often. Also, it should be possible to set up periodic builds for master.