Remove the currently_met field from a Need

This question on the form is redundant as it is 
confusing most of our users in testing, and the 
users which actually require this information 
already capture it through another process in the 
transition project.
Add the mongoid_rails_migrations gem
Add a migration to unset the currently_met field
Merge pull request #20 from alphagov/remove-currently-met

Remove the currently_met field from a Need