only enable source annotation on rails >= 4.2
only enable source annotation on rails >= 4.2

Rails 3.2 .. 4.1 already supported HAML. Also, the original previous
commit would have broken on Rails < 4.2.

olivierlacan/haml-rails,ipmobiletech/haml-rails,indirect/haml-rails,charly/haml-rails,indirect/haml-rails,olivierlacan/haml-rails,ipmobiletech/haml-rails,charly/haml-rails