Fixed issue with xargs on Mac OS X.
Fixed issue with xargs on Mac OS X.

Removed cleaning of reports folders, since this was
interferring with coverage reports generated
sequentially by rake test.

Stanford-Online/edx-platform,jbassen/edx-platform,jbassen/edx-platform,Stanford-Online/edx-platform,Stanford-Online/edx-platform,caesar2164/edx-platform,jbassen/edx-platform,caesar2164/edx-platform,caesar2164/edx-platform,jbassen/edx-platform,caesar2164/edx-platform,Stanford-Online/edx-platform,jbassen/edx-platform