SPARK-1455: Better isolation for unit tests.

This is a simple first step towards avoiding running the Hive tests
whenever possible.

Author: Patrick Wendell <pwendell@gmail.com>

Closes #420 from pwendell/test-isolation and squashes the following commits:

350c8af [Patrick Wendell] SPARK-1455: Better isolation for unit tests.
(cherry picked from commit 5aaf9836f108d4ef9afe809353ad4d3aed560368)

Signed-off-by: Patrick Wendell <pwendell@gmail.com>
[FIX] update sbt-idea to version 1.6.0

I saw `No "scala-library*.jar" in Scala compiler library` error in IDEA. It seems upgrading `sbt-idea` to 1.6.0 fixed the problem.

Author: Xiangrui Meng <meng@databricks.com>

Closes #419 from mengxr/idea-plugin and squashes the following commits:

fb3c35f [Xiangrui Meng] update sbt-idea to version 1.6.0
(cherry picked from commit 8517911efb89aade61c8b8c54fee216dae9a4b4f)

Signed-off-by: Patrick Wendell <pwendell@gmail.com>