SPARK-2899 Doc generation is back to working in new SBT Build.

The reason for this bug was introduciton of OldDeps project. It had to be excluded to prevent unidocs from trying to put it on "docs compile" classpath.

Author: Prashant Sharma <prashant.s@imaginea.com>

Closes #1830 from ScrapCodes/doc-fix and squashes the following commits:

e5d52e6 [Prashant Sharma] SPARK-2899 Doc generation is back to working in new SBT Build.