Update the travis configurations to only build the "master" branch or…

… any branches prefixed with "release" in order to reduce the duplicate use of build bots on travis for pull requests based on non-user-fork-originated branches.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=116275195

.travis.yml

@@ -25,6 +25,8 @@ env:
     - secure: "bTcwsovwxPXplZysfwgNkTR3hfHjb7UvWMlxeEkHHt3GQiZxIDKkiJbgW2mHAG/e/H0wfKQyujeCgQwxn1fa5ttR+UbGz+TIIY2tgjpIFkSbBRzlNGOO0Y23wQpFXXUv3lAY//cV1pa0HlCz+IWNq7ZqPZAoReDAkxExbbmydtE="
     - secure: "JZnVEfpNSCLBZQg1MP7MuhzP9H8t2gGUU4salm5VsRKck27fgg1HwBxADolcVeON2k+2masSKLEQPkeYQizc/VN5hZsCZpTgYjuMke1ZLe1v0KsIdH3Rdt77fhhTqiT1BEkMV8tlBwiraYZz+41iLo+Ug5yjgfmXXayDjYm4h4w="
 
+
 branches:
-  except:
-    - gh-pages
+  only:
+    - master
+    - /^release.*$/