Another handful of small build changes to organize and standardize a bit, and avoid warnings:

- Update Maven plugin versions for good measure
- Since plugins need maven 3.0.4 already, require it explicitly (<3.0.4 had some bugs anyway)
- Use variables to define versions across dependencies where they should move in lock step
- ... and make this consistent between Maven/SBT

OK, I also updated the JIRA URL while I was at it here.

Author: Sean Owen <sowen@cloudera.com>

Closes #291 from srowen/SPARK-1387 and squashes the following commits:

461eca1 [Sean Owen] Couldn't resist also updating JIRA location to new one
c2d5cc5 [Sean Owen] Update plugins and Maven version; use variables consistently across Maven/SBT to define dependency versions that should stay in step.