for edxapp, install python sandbox requirements into regular venv

temporary fix to allow some things that depend on this (like
drag-and-drop questions) to not break.  Also, this configuration
might be what trial installs prefer, anyway.
Merge pull request #172 from edx/jbau-temporary-install-sandbox-reqs-into-normal-venv

for edxapp, install python sandbox requirements into regular venv