don't try to bootstrap if Haml constant is not defined

this handles the "rake gems:install" case where the
application and plugins load, but not gems

init.rb

@@ -1,9 +1,12 @@
-Haml::init_rails(binding) unless defined? Haml::Template
-require 'standardista'
-require 'standardista/implicit_tags'
+if defined? Haml
+  Haml::init_rails(binding) unless defined? Haml::Template
 
-config.after_initialize do
-  unless :xhtml == Haml::Template::options[:format]
-    require 'standardista/rails_override'
+  require 'standardista'
+  require 'standardista/implicit_tags'
+
+  config.after_initialize do
+    unless :xhtml == Haml::Template::options[:format]
+      require 'standardista/rails_override'
+    end
   end
 end