Only run ci/after_success.sh, if a PR is merged

This should do the following:
1. Not run the code in `ci/after_success.sh` when a pull request is
   built, as the secret variable `GH_TOKEN` is not available then, which
   will cause a build error.
2. Run said script after a pull request has been merged to master, to
   automatically update documentation on gh-pages.

I know that 1. actually happens. I have no idea about 2.

Author: hannobraun

ci/after_success.sh

@@ -15,6 +15,6 @@ main() {
         echo OK
 }
 
-if [ $TRAVIS_BRANCH = master ]; then
+if [ "$TRAVIS_EVENT_TYPE" == "push" ] && [ "$TRAVIS_BRANCH" == "master" ]; then
     main
 fi