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/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