Avoid double builds for tags

This is pretty sad, as I'd like for build status to also be visible for
release tags, but
https://github.com/travis-ci/travis-ci/issues/2456#issuecomment-441393704
and
https://github.com/travis-ci/travis-ci/issues/2200#issuecomment-441395545
make that unreasonable. So for the time being we'll settle with just
doing CI for PRs and pushes to master.

.travis.yml

@@ -5,9 +5,10 @@ rust:
   - beta
   - nightly
 
-# only test master and tagged releases on push
 # always test things that aren't pushes (like PRs)
-if: type != push OR branch = master OR branch =~ /^v\d+\.\d+(\.\d+)?(-\S*)?$/
+# never test tags or pushes to non-master branches (wait for PR)
+# https://github.com/travis-ci/travis-ci/issues/2200#issuecomment-441395545)
+if: type != push OR (tag IS blank AND branch = master)
 
 before_script:
  - if [[ "$INTEGRATION" == "true" ]]; then