activerecord: add missing require statements

I have a project that users ActiveRecord without the rest of rails, and
without bundler. When upgrading from 6.0 to 6.1, I started getting
`unitialized constant ..::URI` errors.

Author: terceiro

activerecord/lib/active_record/database_configurations.rb

@@ -1,5 +1,6 @@
 # frozen_string_literal: true
 
+require "uri"
 require "active_record/database_configurations/database_config"
 require "active_record/database_configurations/hash_config"
 require "active_record/database_configurations/url_config"

activerecord/lib/active_record/database_configurations/connection_url_resolver.rb

@@ -1,5 +1,6 @@
 # frozen_string_literal: true
 
+require "uri"
 require "active_support/core_ext/enumerable"
 
 module ActiveRecord