update activerecord to version 6.1

[bishwahang]

I have updated the activerecord version to 6.1 and made changes accordingly to fix the warnings.

I'm not sure how the releases work, but I guess this might be a breaking release. I cannot see the test matrix for other versions in travis.

We can add some guard clauses around the new changes based on activerecord version.

• use connection_db_config for config hash
• update sqlite3 gem
• add gemfiles and test matrix for 6.1

[bishwahang]

Should have checked this https://github.com/influitive/apartment/pull/665 . Closing this PR