Update travis to specify specific branches to run.

This moves the Travis CI config inline with the rest of the RSpec repos.
This will also help reduce the load on the Travis system for rspec-rails
PRs when the PR branch is a branch in the repo itself.

Author: cupakromer

.travis.yml

@@ -89,3 +89,8 @@ matrix:
     - rvm: 2.2
       env: RAILS_VERSION='~> 3.1.12'
   fast_finish: true
+
+branches:
+  only:
+    - master
+    - /^\d+-\d+-maintenance$/