Support for contracts is a development-only mechanism which impacts performance.
It produces a warning on each run that is not easy to act on for our users,
so patch this out altogether.

--- a/lib/nanoc/core/contracts_support.rb	2021-07-11 08:54:09.979327770 +0200
+++ b/lib/nanoc/core/contracts_support.rb	2021-05-28 11:33:52.000000000 +0200
@@ -90,8 +90,7 @@
 
         contracts_loadable =
           begin
+            false
-            require 'contracts'
-            true
           rescue LoadError
             false
           end