Cut down the granularity of travis tests.

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 apache#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.

.travis.yml

@@ -20,18 +20,13 @@
    - oraclejdk7
  env:
   matrix:
-   - TEST=sql/test
+   - TEST="scalastyle assembly/assembly"
+   - TEST="catalyst/test sql/test streaming/test mllib/test graphx/test bagel/test"
    - TEST=hive/test
-   - TEST=catalyst/test
-   - TEST=streaming/test
-   - TEST=graphx/test
-   - TEST=mllib/test
-   - TEST=graphx/test
-   - TEST=bagel/test
  cache:
    directories:
      - $HOME/.m2
      - $HOME/.ivy2
      - $HOME/.sbt
  script:
-   - "sbt ++$TRAVIS_SCALA_VERSION scalastyle $TEST"
+   - "sbt ++$TRAVIS_SCALA_VERSION $TEST"