Disable travis-ci branch builds for most branches

(We really don't need to kick off 10 builds for the branch as well as 10 for
the PR)

.travis.yml

@@ -4,6 +4,12 @@ language: python
 # tell travis to cache ~/.cache/pip
 cache: pip
 
+# only build branches we care about (PRs are built seperately)
+branches:
+  only:
+    - master
+    - develop
+
 before_script:
   - git remote set-branches --add origin develop
   - git fetch origin develop