Add 1/(xy) to acceptable formats

Summary:
acceptable formats for expressions is becoming unweildly :(
I'm not sure this is the best way to address this particular confusing,
but am interested in hearing other people's thoughts.

Test Plan:
Er... acceptable formats css has been broken sometime after the last
subperseus, so while this works, it still looks horribly ugly (but
doesn't seem to be the cause of the ugliness)

Reviewers: eater, brianmerlob

Reviewed By: brianmerlob

Subscribers: brianmerlob, alpert

Differential Revision: http://phabricator.khanacademy.org/D7436