kiji-schema: SCHEMA-628. Re-enable kiji-schema-extras module

 - Untangle Java/Scala inter-dependencies.

issue: https://jira.kiji.org/browse/SCHEMA-628
review: https://review.kiji.org/r/1574
commit: https://github.com/kijiproject/kiji-schema/commit/086b80e63468419af36512cb5b1e567d2cab46ef