Travis CI: Use apt-get to install WebP instead of building from source

Travis CI has introduced ["per-command time tags"](http://blog.travis-ci.com/2014-08-13-per-command-time-tags/) which show how long each command takes.

From this, we can see WebP installation takes just under a minute per job.

WebP is downloaded and built from source, but can we just `apt-get install` it following [Pillow's docs](http://pillow.readthedocs.org/en/latest/installation.html#linux-installation)?

If so, the install takes under 3 seconds and the entire build drops from around 50 minutes total to 40 minutes.