Update ruby-dm-core package to 1.2.0.

Update ruby-dm-core package to 1.2.0.

* STI queries no longer include the top-level class name
* UnderscoredAndPluralizedWithoutLeadingModule naming convention was added
  belongs_to supports :unique option
* Validation of property names was improved
* Resource[] and Resource[]= no longer fail when property name is not known
* Redundant usage of chainable was removed resulting in a better performance
* Boolean property typecasting was refactored
* Various issues with setting default Property options were fixed
* Resource#attributes= no longer use public_method_defined? - this is a
  security fix preventing possible DDOS attacks
* Problems with auto-migrations in multiple repositories were fixed
* Encoding problems with Binary property are fixed