Configure Travis to push multiple versions to GH Pages

[IanMayo]

The first step of our build process will be to push the current version to gh-pages.

A more refined version of this is to push new builds to time-stamped folders.

I guess it needs a process like this:

• generate timestamp
• to build
• create folder with timestamp name in gh-pages branch
• push build into this folder
• append link to new index.html to a top-level index.html, annotated with PR name.