Add support for Travis doing svn-push on plugins after_success

[westonruter]

By using encrypted environment variables in Travis, we can store WordPress.org SVN credentials directly in the .travis.yml file. In the Travis after_success config, we can then automatically do a commit to SVN for the plugin. We'll have to rework things to tag releases on WordPress.org. We could pick up the tag push on GitHub by webhook and automatically supply this for the stable tag in the plugin's SVN trunk. But then we'd need to figure out how to handle the changelog.

[valendesigns]

I think we should find a way to limit svn_push to when a tag is created on master or some other manual indication so it doesn't push every commit to master up to svn. It's especially important for those repos that haven't yet adopted a clear branching strategy and all commits and PR's are being sent to master.