This PR amortizes the cost of downloading all the jars and compiling core across more test cases.  In one anecdotal run this change takes the cumulative time down from ~80 minutes to ~40 minutes.

Author: Michael Armbrust <michael@databricks.com>

Closes #255 from marmbrus/travis and squashes the following commits:

506b22d [Michael Armbrust] Cut down the granularity of travis tests so we can amortize the cost of compilation.