Add support for Rails 6.1 (another version)

[hgwr]

This is another version PR of #77

If we prefer #77, please close this PR.

To resolve #75, this PR fixes some errors and warnings caused from Rails 6.1. Roughly the same with #70.

DEPRECATION WARNING: [] is deprecated and will be removed from Rails 6.2 (Use configs_for)

DEPRECATION WARNING: DatabaseConfig#config will be removed in 6.2.0 in favor of DatabaseConfigurations#configuration_hash which returns a hash with symbol keys

Note:

• On Rails 6.1 environment, we need to add selenium-webdriver gem
• We success to bundle exec rake test on both of Rails 6.0 / 6.1 environment.

[hgwr]

I forgot to add Rails 6.2 support, so I'm closing this PR. I'm sorry for the inconvenience.