update automated build s/t site publishes to gh-pages branch instead of master

[capsulecorplab]

In lieu of https://github.com/github/renaming, GitHub has removed the hardcoded dependency for users and organizations to publish to the master branch. See Build and deploy GitHub Pages from any branch beta. This issue entails renaming the default publishing branch from master to gh-pages, for sake of consistency with repos that already publish to gh-pages