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.
Model validations for Kalibro Configuration.

Also removed unnecessary method.
Model validations for Reading Group.
Model validations for Reading.
Model validations for Metric Configuration.

Created association with Metric.
WIP: Model validations for Metric.

What is left to be done?
- Test validations with conditionals.
- Validates uniqueness of code in the scope of a kalibro_configuration,
  not in the scope of a metric_configuration.
Added validator on MetricConfiguration to check code uniqueness

Signed off by: Heitor Reis <marcheing@gmail.com>
Added validators on KalibroRanges

Signed off by: Heitor Reis <marcheing@gmail.com>
Added dependency validators

Signed off by: Heitor Reis <marcheing@gmail.com>