Update a series of gem dependencies

* Update the ruby version in the Gemfile to `2.1.5`.
* Updated the `devise` dependency to the latest release.
* Removed the `mail_form` dependency as it is not used by the application anymore.
* Updated a few dependencies that were safe to update - `jquery-rails`, `responders`, etc.
* Updated several test/development gems to their last versions - `rspec` is still on the 2.99 version to avoid any extra legwork with the upgrades. 

Ideally after these updates we can try to update other dependencies that are more crucial to the app (like the `omniauth` family or `rspec-rails` and the spec suite) before jumping to Rails 4.0.12.

Everything seems to be green, but I still want to boot the app locally and take it for some manual tests. Each upgrade has its own commit, but I can squash ~17 commits into a single *Update all the things* changeset if necessary.