Enable preview processing on Travis CI
Propagating tests exit code in Grunt
Removed 'excluding pp tests' in README.md
Add a note about etherpad-lite @ 1.2.91 in the README
Added a test that verifies previews can be reprocessed with a revision filter
Try to speed up previews build and reduce failures by:

 * Reduce timeouts
 * Purge all previews queues before each test
 * Installing etherpad package