Add support for Django 2.1

[Surgo]

Subject: Add support for Django 2.1

Feature or Bugfix

• Feature
• Bugfix

Purpose

• Fixes #49
• Some changes for that backward impompatible changes

Detail

• To adhere to PEP 249, exceptions where a database doesn’t support a feature are changed from NotImplementedError to django.db.NotSupportedError.
• DatabaseOperations.distinct_sql() now requires an additional params argument and returns a tuple of SQL and parameters instead of a SQL string. Thank you @jdlourenco

Relates

• 49

• https://docs.djangoproject.com/en/2.1/releases/2.1/

[shimizukawa]

Thanks! I'll take a look in this weekend.

[Surgo]

Django 2.1 env was added on travis :)

[shimizukawa]

Oh, I noticed the [WIP] sign just now. Should I wait review yet?

[Surgo]

Oh, sorry... #57 is WIP :(

[Surgo]

@shimizukawa Fixed commented issues.

[shimizukawa]

Thanks! I'll take a look.