Update README Postgres example for Ruby 3.1

[duffn]

The current example only works with pre 3.1 and pre Psych 4. Upgrade it to work with all versions.

• Ref: https://bugs.ruby-lang.org/issues/17866
• Ref: https://github.com/rails/rails/commit/179d0a1f474ada02e0030ac3bd062fc653765dbe

[dblock]

I don't love relying on an exception. Can we do an explicit version check please? Gem::Version.new(...) >= ....

[duffn]

Updated.