remove ruby-filemagic since heroku can't install it.

filemagic is just ruby bindings to a library - libmagick. This library
isn't installed on heroku, nor do we have the ability to install it on
heroku. So we're going to make the assumption that only jpeg will be
sent to the api.
Merge pull request #12 from wetap/feature/fix_mime_type_for_heroku

remove ruby-filemagic since heroku can't install it.