Fix cleanup .pyc file command in rake test

When Python files were pre-compiled at the top level of the edx-platform repo, `rake test` would fail with this error:

    find . -type f -name *.pyc -delete
    find: paths must precede expression: setup.pyc

The issue was that the shell was expanding `*.pyc` instead of using it as a wildcard match in the find.  This PR fixes the issue by quoting `*.pyc` to prevent shell expansion.

@jzoldak