Upgrade edx-platform to version of xblock from https://github.com/edx/XBlock/pull/240.

@lukedmor Would it be possible for you to look at these test failures? When I include your commit (https://github.com/edx/XBlock/pull/240) in edx-platform, some unit tests are failing.

I am in the process of merging a change to XBlock, and I will need to include it in edx-platform.

FYI: @nedbat