Add support for Postgres array and hstore types

[mansakondo]

Related #16

Worth mentioning

• This PR should probably be split into multiple PRs, as I added support for running tests with Postgres and Rails 7.
• There's an issue with Rails < 4.2.5 and Postgres 12 so it might be necessary to drop support for Rails 4.0 and 4.1.
• JSON and JSONB types can also be supported by adding them to the DEFAULT_TYPES.
• The travis.yml file needs to be updated.