Remove the dependency on JUnit from SamzaTaskManager

Remove the dependency on JUnit, which shouldn't be in production code and in some environments, could be explicitly excluded and cause build errors.

Interestingly, gradle allowed the dependency even though we only ever add JUnit as a testCompile dependency. It looks like this is due to a transitive dependency.

Snippet from the command "./gradlew :samza-yarn:dependencies":
compile - Compile classpath for source set 'main'.
...
--- org.apache.zookeeper:zookeeper:3.3.4
+--- log4j:log4j:1.2.15 -> 1.2.17
--- jline:jline:0.9.94
--- junit:junit:3.8.1