Exclude tagged branches from travis builds

Tagged branches are created by successful builds in our CI environment.
It is wasteful and potentially clogs up our build pipeline in travis for
pull requests if we also build those versions in travis.

.travis.yml

@@ -16,5 +16,6 @@ after_script:
 branches:
   except:
     - release
+    - /^release_[0-9]+$/
 notifications:
   email: false