Should validate ruby, db, and rails values, know exclusions

[ronen]

Should have knowledge of which values of ruby, db, and rails are supported at all by any gem in the schema_plus family, and should validate the values in schema_dev.yml

Should know about exclusions that all gems have; auto-generate those exclusions in the matrix. (Specific client gems could add extra exclusions).

Should have some way to override when developing for previously-unsupported versions of ruby, rails, and/or db.