Duplicate interactive titles

Enabled Rails model validations during the import of interactives and models from interactives.json. 
This flagged a problem with two interactives within the same group having the same title. 

PT Story:
https://www.pivotaltracker.com/s/projects/731349/stories/49000961

Check the 'Draft*' checkbox at http://lab3.dev.concord.org/interactives

Notice that there are now two Phase Change interactives in the group "Science of Atoms and Molecules: Phase Change" on the select Interactive dropdown. One named, "Phase Changes with two bar graphs" and one named "Phase Changes".

Previously, only one Phase Change interactive was being served because of this bug.

Deployed on lab3.dev.concord.org