Don't use MONGO_URI env var in development
Don't use MONGO_URI env var in development

Without this change, the app connects to different databases, depending
on whether it's started through `bowler` or through `bundle exec rails s`,
which was causing grief.

alphagov/specialist-publisher,alphagov/specialist-publisher-rebuild,alphagov/specialist-publisher-rebuild,alphagov/specialist-publisher,alphagov/specialist-publisher,alphagov/specialist-publisher-rebuild,alphagov/specialist-publisher-rebuild