Remove uneeded versioned api paths

* Hypermedia yawl
Add categories table to db
Add Category model
Use pg for all environments
Install foreigner gem
Add categorizations table to db
Implement Categorization model
Add Category associations to Article
Add category to v1 articles endpoint
Add categories to v2 articles endpoint
Extend update spec to check article updated
Categories may be included in article creation and update
Merge branch 'categories'