Enable gradients when running with Prawn 2.2.0+

Closes #62.

lib/prawn/svg/elements/gradient.rb

@@ -42,12 +42,9 @@ def type
   end
 
   def assert_compatible_prawn_version
-    # At the moment, the patch required for this functionality to work in prawn has not been merged.
-    raise SkipElementError, "We are unfortunately still waiting on the Prawn project to merge a pull request that is required for this feature to correctly function"
-
-#    if (Prawn::VERSION.split(".").map(&:to_i) <=> [2, 0, 4]) == -1
-#      raise SkipElementError, "Prawn 2.0.4+ must be used if you'd like prawn-svg to render gradients"
-#    end
+    if (Prawn::VERSION.split(".").map(&:to_i) <=> [2, 2, 0]) == -1
+      raise SkipElementError, "Prawn 2.2.0+ must be used if you'd like prawn-svg to render gradients"
+    end
   end
 
   def load_gradient_configuration