Give a better error message to users with an unsupported version of haml installed.

lib/compass.rb

@@ -1,5 +1,19 @@
 require 'rubygems'
 require 'sass'
+
+def assert_sass_version(obj)
+  unless obj.respond_to?(:version) && obj.version[:major] == 2 && obj.version[:minor] >= 1
+    raise LoadError.new("Compass requires Haml version 2.1 or greater.")
+  end
+end
+
+begin
+  assert_sass_version(Sass)
+rescue LoadError
+  require 'haml'
+  assert_sass_version(Haml)
+end
+
 require File.join(File.dirname(__FILE__), 'sass_extensions')
 
 ['core_ext', 'version'].each do |file|