Add migration to convert color column to string

The c205ed62 commit changed this directly on the schema, but
everytime a migration was run, it reverted the field back to
integer and added a configurations column. Hopefully, this
commit fixes this.
Null session API
Created index action for kalibro_ranges_controller

- This commit removes the action ranges_of from
  metric_configurations_controller and adds a index action to
  kalibro_ranges_controller that does the same.
- The route moved to /metric_configurations/:id/ranges_of/ to
  /metric_configurations/:id/kalibro_ranges/
Created index action for readings_controller

- This commit removes the action readings_of from
  reading_group_controller and adds a index action to
  readings_controller that does the same.
- The route moved to
  /reading_groups/:id/readings_of/ to
  /reading_groups/:id/readings/