added `jsonb` datatype to PostgresAdapter

This fixes issues with schema dumping when using the `jsonb` datatype for Postgres. Included in this PR:
* adding `jsonb` to `NATIVE_DATABASE_TYPES`
* adding table/column creation for JSONB when testing with Postgres
* Formatting and getting `postgresql_specific_schema.rb` to 120 wide
let's reword the CHANGELOG entry to state that it fixed schema dumping. Something along the lines of:

> Added SchemaDumper support for tables with jsonb columns.