Check Rails.version instead of ENV

Like that we can have code that works on multiple Rails versions, and we
dont need to mix a new method on Kernel.
Also, this makes easier to have multiple versions.
For instance, before master was 4.2, which is not the case anymore, so
on the code we should check versions and not Environment variables

config/application.rb

@@ -117,7 +117,7 @@ module Discourse
     # see: http://stackoverflow.com/questions/11894180/how-does-one-correctly-add-custom-sql-dml-in-migrations/11894420#11894420
     config.active_record.schema_format = :sql
 
-    if rails_master?
+    if Rails.version >= "4.2.0"
       # Opt-into the default behavior in Rails 5
       # config.active_record.raise_in_transactional_callbacks = true
     end

config/initializers/00-rails-master-polyfills.rb

@@ -1,8 +1,6 @@
-unless rails_master?
-
-class Mail::Message
-  alias_method :deliver_now,  :deliver
-  alias_method :deliver_now!, :deliver!
-end
-
+if Rails.version < "4.2.0"
+  class Mail::Message
+    alias_method :deliver_now,  :deliver
+    alias_method :deliver_now!, :deliver!
+  end
 end

lib/freedom_patches/pool_drainer.rb

@@ -1,5 +1,5 @@
 
-if rails_master?
+if Rails.version >= "4.2.0"
   class ActiveRecord::ConnectionAdapters::AbstractAdapter
     module LastUseExtension
       attr_reader :last_use

lib/sql_builder.rb

@@ -72,7 +72,7 @@ class SqlBuilder
   #AS reloads this on tests
   remove_const :FTYPE_MAP if defined? FTYPE_MAP
 
-  if rails_master?
+  if Rails.version >= "4.2.0"
     FTYPE_MAP = {
       23 => ActiveRecord::Type::Integer.new,
       1114 => ActiveRecord::Type::DateTime.new,
@@ -102,7 +102,7 @@ class SqlBuilder
       setters.each_with_index do |mapper, index|
         translated = row[index]
         if mapper[1] && !translated.nil?
-          if rails_master?
+          if Rails.version >= "4.2.0"
             translated = mapper[1].type_cast_from_database(translated)
           else
             translated = ActiveRecord::ConnectionAdapters::Column.send mapper[1], translated

spec/mailers/user_notifications_spec.rb

@@ -114,7 +114,7 @@ describe UserNotifications do
       response.user.mailing_list_mode = true
       mail = UserNotifications.user_replied(response.user, post: response, notification: notification)
 
-      if rails_master?
+      if Rails.version >= "4.2.0"
         expect(mail.message.class).to eq(ActionMailer::Base::NullMail)
       else
         expect(mail.class).to eq(ActionMailer::Base::NullMail)
@@ -123,7 +123,7 @@ describe UserNotifications do
       response.user.mailing_list_mode = nil
       mail = UserNotifications.user_replied(response.user, post: response, notification: notification)
 
-      if rails_master?
+      if Rails.version >= "4.2.0"
         expect(mail.message.class).not_to eq(ActionMailer::Base::NullMail)
       else
         expect(mail.class).not_to eq(ActionMailer::Base::NullMail)
@@ -196,7 +196,7 @@ describe UserNotifications do
     UserNotifications.any_instance.expects(:build_email).with(user.email, condition)
     mailer = UserNotifications.send(mail_type, user, notification: notification, post: notification.post)
 
-    if rails_master?
+    if Rails.version >= "4.2.0"
       # Starting from Rails 4.2, calling MyMailer.some_method no longer result
       # in an immediate call to MyMailer#some_method. Instead, a "lazy proxy" is
       # returned (this is changed to support #deliver_later). As a quick hack to