Get iHat specs passing

### References:
https://www.pivotaltracker.com/story/show/82063658

* Pulls in VCR to prevent tests from making live requests to Oxgarage
* Updates Gemfile to use locked version of the TahiEpub gem
* Updates the Readme

### Notes
We bumped Faraday down to 0.8.9 in order to work around a VCR bug with Faraday v0.9.0: https://github.com/vcr/vcr/pull/439 (This has been merged into VCR master but isn't in the gem yet).