Changed prereq installation in rake test and rake fasttest

* Ensures prereqs are installed before running asset collection, which otherwise will fail (e.g. if XBlock isn't installed)
* `fasttest` rake tasks will no longer install prereqs
* Adds prereq installation to `test common/lib/xmodule` which didn't have it before.

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