[Spark QA] only check code files for new classes

Look only at code files (`.py`, `.java`, and `.scala`) for new classes.

Should get rid of false alarms like [the one reported here](https://github.com/apache/spark/pull/2014#issuecomment-52912040).

Author: Nicholas Chammas <nicholas.chammas@gmail.com>

Closes #2184 from nchammas/jenkins-ignore-noncode and squashes the following commits:

33786ac [Nicholas Chammas] break up long line
3f91a14 [Nicholas Chammas] rename array of source files
8b82a26 [Nicholas Chammas] [Spark QA] only check code files for new classes