SPARK-2749 [BUILD]. Spark SQL Java tests aren't compiling in Jenkins' Maven builds; missing junit:junit dep

The Maven-based builds in the build matrix have been failing for a few days:

https://amplab.cs.berkeley.edu/jenkins/view/Spark/

On inspection, it looks like the Spark SQL Java tests don't compile:

https://amplab.cs.berkeley.edu/jenkins/view/Spark/job/Spark-Master-Maven-pre-YARN/hadoop.version=1.0.4,label=centos/244/consoleFull

I confirmed it by repeating the command vs master:

`mvn -Dhadoop.version=1.0.4 -Dlabel=centos -DskipTests clean package`

The problem is that this module doesn't depend on JUnit. In fact, none of the modules do, but `com.novocode:junit-interface` (the SBT-JUnit bridge) pulls it in, in most places. However this module doesn't depend on `com.novocode:junit-interface`

Adding the `junit:junit` dependency fixes the compile problem. In fact, the other modules with Java tests should probably depend on it explicitly instead of happening to get it via `com.novocode:junit-interface`, since that is a bit SBT/Scala-specific (and I am not even sure it's needed).

Author: Sean Owen <srowen@gmail.com>

Closes #1660 from srowen/SPARK-2749 and squashes the following commits:

858ff7c [Sean Owen] Add explicit junit dep to other modules with Java tests for robustness
9636794 [Sean Owen] Add junit dep so that Spark SQL Java tests compile
[SPARK-2758] UnionRDD's UnionPartition should not reference parent RDDs
Clear RDDs for checkpointing.
[SPARK-2585] Remove special handling of Hadoop JobConf.