Update README Postgres example for Ruby 3.1 (#170)

* Update README Postgres example for Ruby 3.1

* Update command to load config

* Take YAML loading from tests

Author: duffn

README.md

@@ -101,13 +101,13 @@ production:
 Establish a connection in your startup code.
 
 ```ruby
-ActiveRecord::Base.establish_connection(
-  YAML.safe_load(
-    ERB.new(
-      File.read('config/postgresql.yml')
-    ).result, [], [], true
-  )[ENV['RACK_ENV']]
-)
+yml = ERB.new(File.read(File.expand_path('config/postgresql.yml', __dir__))).result
+db_config = if Gem::Version.new(Psych::VERSION) >= Gem::Version.new('3.1.0.pre1')
+              ::YAML.safe_load(yml, aliases: true)[ENV['RACK_ENV']]
+            else
+              ::YAML.safe_load(yml, [], [], true)[ENV['RACK_ENV']]
+            end
+ActiveRecord::Base.establish_connection(db_config)
 ```
 
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