travis: avoid double builds on pull requests

Build only commits on master and release tags for the
"Build pushed branches" feature.

This prevents building twice on PRs originating from our repo
("Build pushed pull requests)".

Change-Id: Icdf54fb6ebbb67222cc55c624d4ba0e284650e60

.travis.yml

@@ -1,5 +1,16 @@
 ---
 language: python
+
+# Build only commits on master and release tags for the "Build pushed branches" feature.
+# This prevents building twice on PRs originating from our repo ("Build pushed pull requests)".
+# See:
+#   - https://github.com/travis-ci/travis-ci/issues/1147
+#   - https://docs.travis-ci.com/user/pull-requests/#double-builds-on-pull-requests
+branches:
+  only:
+    - master
+    - /v[0-9]+\.[0-9]+\.[0-9]+(-.*)?$/
+
 cache:
   - pip
   - directories: