Fix facebook avatars for future users

Currently when a user creates an account with Facebook, Paperclip throws the following error:

`[paperclip] Content Type Spoof: Filename picture ([]), content type discovered from file command: image/jpeg. See documentation to allow this combination.`

This seems to be the fix based on the issue here: https://github.com/thoughtbot/paperclip/issues/1429

Will close #77, and #86