use postgresql enum as rails column type

[ronen]

Now that schema_plus supports easy definition of enum types in postgresl (#167, thanks @juike), it would be nifty if any enum type could then be used as a column in migrations. See, e.g., discussion at https://coderwall.com/p/azi3ka

[ronen]

Closed in favor of SchemaPlus/schema_plus_enums#2