Add Rails 6.1 support

[ilvez]

An attempt to add Rails 6.1 support. I'm not really sure about the changes, just took e8972ce as an example and made changes that seemed reasonable. Not sure about version specific compatibility need (using 'active_record_upsert/compatibility/rails60.rb'), but my local tests passed.

[ilvez]

So there is big requirements change in postgresql_adapter.rb in activerecord. rails/rails@592358e

In 6.0.3.4 (activerecord-6.0.3.4/lib/active_record/connection_adapters/postgresql_adapter.rb):

# Make sure we're using pg high enough for type casts and Ruby 2.2+ compatibility
gem "pg", ">= 0.18", "< 2.0"
require "pg"

In 6.1.0 (activerecord-6.1.0/lib/active_record/connection_adapters/postgresql_adapter.rb):

gem "pg", "~> 1.1"
require "pg"

[olleolleolle]

I think this looks good, and it was the right call to skip adding a not-used compat Ruby file for the 6.1 version.