CI: only deploy on push TRAVIS_EVENT_TYPE

Merges manubot/rootstock#184

Previously, ci/deploy.sh could be run for push, api, or cron builds,
but not for pull_request builds. This commit further restricts when
deployment is run to just push builds.

.travis.yml

@@ -24,5 +24,5 @@ cache:
     - ci/cache
 after_success:
   - test $TRAVIS_BRANCH = "master" &&
-    test $TRAVIS_PULL_REQUEST = "false" &&
+    test $TRAVIS_EVENT_TYPE = "push" &&
     sh ci/deploy.sh