Updated Gemfile and Deploy instructions

The gems were too out-of-date to let me install (problems with sass, i18n, others), and so I updated them in the Gemfile and Gemfile.lock. The `pry-debugger` gem I never could get to work locally -- it's removed. The other version constraints were added because of incompatibilities.

Deploy instructions now have a snippet to make your key for you, and modernize to match the new Heroku toolbelt syntax. Also added the db:seed step, necessary to get the extra agreement fields named.