Upgrade MLlib build to not depend on SNAPSHOT versions; fix name of MLlib test assembly JAR

Author: Josh Rosen <joshrosen@databricks.com>

Closes #42 from JoshRosen/mllib-spark-versions-fix and squashes the following commits:

c5b9b46 [Josh Rosen] Use correct name for mllib test assembly JAR
6d7a2c7 [Josh Rosen] Remove SNAPSHOT versions from MLlib tests build; re-enable Travis for MLlib tests