Install edx-submissions from PyPi instead of from edx-ora2

Companion PR to https://github.com/edx/edx-ora2/pull/411 in the edx-ora2 repo.
This installs the `submissions` app from PyPi instead of through edx-ora2.

Once the edx-ora2 PR gets merged, I'll update the edx-ora2 commit in the edx-platform reqs.

@ormsbee