Changed prereq installation in rake test and rake fasttest

* Now uses `install_prereqs` exclusively instead of `predjango` (before it used both, resulting in redundant pip calls)

* `fasttest_*` commands no longer do any prereq installation

This resolves an issue that was blocking my Jenkins cluster work: `rake test_acceptance` fails on asset compilation if XBlock hasn't been installed, but it won't install XBlock until asset compilation succeeds.

Reviewers: @cpennington @jzoldak