Update to XBlock version that installs workbench packages.

Let's see if this unbreaks Travis. Note that if you've got a local XBlocks install, you'll want to `pip uninstall xblock` first. I actually had to do it twice, but that may just have been as a result of my mucking around locally with invoking setup.py in different ways. In any case, after pip tells you xblock is no longer installed, run `pip install -r base.txt`