kiji-mapreduce-lib: KIJIMRLIB-30. Update scala maven plugin dependency.

 - Scala-maven-plugin is not longer, has been replaced with net.alchim.
 - Fix compile error in TestSpiContract.scala (not sure why Scala compiler fails...)

issue: https://jira.kiji.org/browse/KIJIMRLIB-30
review: https://review.kiji.org/r/2061
commit: https://github.com/kijiproject/kiji-mapreduce-lib/commit/25a773898b01e0c6dda29dc390e82fc80a85ec60