Only run semantic-release on master branch

As this is an NPM module, the master branch is where we do releases.
Running semantic-release anywhere else just wastes time on TravisCI.

Author: orangejulius

.travis.yml

@@ -19,3 +19,4 @@ jobs:
     - stage: release
       node_js: 10
       script: curl "https://raw.githubusercontent.com/pelias/ci-tools/master/semantic-release.sh" | bash -
+      if: branch = master