Module Related Posts: IE9 Compat Fix

IE9 does not support `history.replaceState()` which breaks rendering of the
related posts section. Abort trying to cleaning up the URL if we cannot
rewrite state. (Sadly this leaves cruft on the URL of older browsers.)
Merge pull request #310 from xyu/module-related-posts-url-cleanup-ie9

Module Related Posts: IE9 Compat Fix