Travis ckan master and other testing fixes

[brew]

This PR fixes a few small testing errors and issues with Travis:

• It bumps the CKAN dependency used by Travis to 2.5 (it was on a ckan feature branch which has now been merged into CKAN master).
• It fixes a test by explicitly setting the site_url so the relevant assert works both locally, and on Travis.
• There's also a fix for issue #3, ensuring httpretty is pinned to an older version stop slow tests, and BadStatusLine errors causes by newer versions.

[pwalsh]

@brew good to merge.