Fix text diffs not expanding appropriately

Summary:
Text diffs in Perseus One would not expand when switched out for an
entry that had differences. This commit makes the text diffs not only
check if they should be expanded on getInitialState, but also on
componentWillReceiveProps.

Test Plan:
* Open difftest.html
* Click the swap button a few times to exchange the diff for one that
  has changed
* Should see it expand and contract appropriately

Reviewers: alex

Reviewed By: alex

CC: alpert

Differential Revision: http://phabricator.khanacademy.org/D7108