add activerecord as a development dependency
add activerecord as a development dependency

Prior to this change, running `bundle install` and then `bundle exec
rspec` produced the error:

`spec/spec_helper.rb:2:in `require': cannot load such file -- active_record (LoadError)`

So, in order to run tests successfully, add `activerecord` as a dev
dependency.

meyouhealth/flexible_enum