Squash stdout/stderr in gh-pages push

Previously if there was a problem communicating to GitHub or if there was an issue with the GH_TOKEN the token would be displayed and stored in Travis. This update hides the token but still indicates the result for Travis.

Tokens are meant to be secret; they should never be logged.

It probably also makes sense to have the script return non-zero on a failed gh-pages push. This would cause the Travis job to fail so I didn't do it because it would change the way the builds work. If others agree then I'll update the pull request.