Bug 1024993 -  Run the CSS Linter in Travis/TBPL

Refactor the script to only report unknown problems
Same comment as above.

If you have a getCSSFilesFor(...) method the first block will be:

files = getCSSFilesFor(webapp.sourceDirectoryFile);

and this one will be:

files = files.concat(getCSSFilesFor(gaia.sharedFolder));