PostgreSQL: Allow pg-1.0 gem to be used with ActiveRecord

pg-1.0.0 is just released and most Gemfiles don't restrict
it's version. But the version is checked when connecting to
the database, which leads to the following error:

Gem::LoadError: can't activate pg (~> 0.18), already activated pg-1.0.0

See also this pg issue:
https://bitbucket.org/ged/ruby-pg/issues/270/pg-100-x64-mingw32-rails-server-not-start

Preparation for pg-1.0 was done in commit f28a331023,
but the pg version constraint was not yet relaxed.

actioncable/lib/action_cable/subscription_adapter/postgresql.rb

@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 
-gem "pg", "~> 0.18"
+gem "pg", ">= 0.18", "< 2.0"
 require "pg"
 require "thread"
 require "digest/sha1"

activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb

@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 
 # Make sure we're using pg high enough for type casts and Ruby 2.2+ compatibility
-gem "pg", "~> 0.18"
+gem "pg", ">= 0.18", "< 2.0"
 require "pg"
 
 require "active_record/connection_adapters/abstract_adapter"

railties/lib/rails/generators/app_base.rb

@@ -301,7 +301,7 @@ module Rails
         # %w( mysql postgresql sqlite3 oracle frontbase ibm_db sqlserver jdbcmysql jdbcsqlite3 jdbcpostgresql )
         case options[:database]
         when "mysql"          then ["mysql2", ["~> 0.4.4"]]
-        when "postgresql"     then ["pg", ["~> 0.18"]]
+        when "postgresql"     then ["pg", [">= 0.18", "< 2.0"]]
         when "oracle"         then ["activerecord-oracle_enhanced-adapter", nil]
         when "frontbase"      then ["ruby-frontbase", nil]
         when "sqlserver"      then ["activerecord-sqlserver-adapter", nil]

railties/test/generators/app_generator_test.rb

@@ -413,7 +413,7 @@ class AppGeneratorTest < Rails::Generators::TestCase
     if defined?(JRUBY_VERSION)
       assert_gem "activerecord-jdbcpostgresql-adapter"
     else
-      assert_gem "pg", "'~> 0.18'"
+      assert_gem "pg", "'>= 0.18', '< 2.0'"
     end
   end