Do not run Travis on every branch, just master

Author: axic

.travis.yml

@@ -11,6 +11,11 @@ addons:
     - ubuntu-toolchain-r-test
     packages:
     - g++-4.8
+branches:
+  # We need to whitelist the branches which we want to have "push" automation.
+  # Pull request automation is not constrained to this set of branches.
+  only:
+  - master
 env:
   matrix:
     - CXX=g++-4.8 TEST_SUITE=test