Change casperjs helper used to get RSS page content.

closes #3431
- change feed_test.js to call casperjs getHTML() instead of
  getPageContent(), because the latter now strips markup

Alternative fix would be to use .page.frameContent directly
instead of using a casperjs wrapper.

Change runs cleanly on Travis (old casperjs tag) and locally
in the Ghost-Vagrant setup with both the old casperjs tag
and the current HEAD SHA (29bbd03f7cdc9fc1e70f0fe1400635dd7d3537ad)