Travis: don't build all branches

Otherwise PRs from people who create branches in the main repo will always
trigger two builds.

The second entry should ensure that we do build releases.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

.travis.yml

@@ -167,6 +167,6 @@ after_success:
     - source ${TRAVIS_BUILD_DIR}/scripts/${SUBSURFACE_PLATFORM}/after_success.sh
 
 branches:
-  except:
-    - # Do not build tags that we create when we upload to GitHub Releases
-    - /^(?i:continuous)/
+  only:
+    - master
+    - /^v\d+\.\d+(\.\d+)?$/