Fix confusing sign up flash message.

Closes #174
jquery-rails 2.2.x breaks fulcrum

jquery-rails 2.2 brings in the jQuery 1.9 series. One change that this
release brings is the removal of the jQuery.browser object. As a
consequence, using jquery-rails 2.2 will cause a TypeError in the
javascript for fulcrum.

This change to the Gemfile cause us to select the most recent release of
jquery-rails on the 2.1.x series, starting with at least v2.1.4
addition of font-awesome to replace tango icons