Fix Parsoid's span.reference styling to match PHP's output

Fix Parsoid's span.reference styling to match PHP's output

* In Chrome (and other webkit-based browsers like phantomjs),
  the computed font-size for span.reference in Parsoid was 12px
  whereas in Firefox, it was 11px. The computed font-size for
  <sup> used for references for core mediawiki css is 11px in
  both Firefox and Chrome. I was able to pin this on the
  font-size: smaller vs font-size: 80% difference by inspecting
  in Chrome and Firefox (and also by dumping computed CSS in
  phantomjs).

* Verified via visual-diffing on ruthenium that this fix cleans
  up the diffs.

  http://parsoid-tests.wikimedia.org/visualdiff-item/pngs/enwiki/Medha_Patkar.diff.png

Change-Id: Ida67035cfb020696d2299ee26fe8d44717ffc421