speed up travis build times by using pip-accel

A few notes:

 * AWS S3 is used as the cache. When travis enables caching for
   free accounts, this dependency can be removed
 * it currently uses credentials for my personal AWS account. I'm
   fine with this as long as costs don't balloon. I'll keep an
   eye on it
 * build time of this PR isn't an indication of the speed-up, since
   the cache is empty