Fix bingo_alternative_selector.py to handle when experiment names have parentheses in them.

Test Plan: Tested rsplit('(', 1)[0].strip() with "blah (blah)" and "blah (blah) (blah)" in a python console. Confirmed that "blah" was returned in the first case, and "blah (blah)" in the second case.

Reviewers: jace, eliana, benkomalo

Reviewed By: benkomalo

CC: benkomalo

Differential Revision: http://phabricator.khanacademy.org/D2153