Only run Travis tests on master and X.Y branches (GH-102)

If someone pushes a branch to python/cpython and then creates a PR
it will cause Travis to run tests needlessly, once for the PR and
once for the push. This will limit the branches that Travis will
run tests for to the `master` branch and branches that match the regex
`^\d\.\d$`.

This will have the effect that if someone purposely makes another branch
they won't get tests to run, but in that rare case they can adjust this
themselves.