Prevent overriding coffee cache once registered

Previously this was done afterwards in index.html and task.coffee
but should really be handled by the cache itself so any module
require a different version of coffee-script are not reregistering
a different require extension handler.

src/coffee-cache.coffee

@@ -34,4 +34,8 @@ requireCoffeeScript = (module, filePath) ->
 
 module.exports =
   cacheDir: cacheDir
-  register: -> require.extensions['.coffee'] = requireCoffeeScript
+  register: ->
+    Object.defineProperty(require.extensions, '.coffee', {
+      writable: false
+      value: requireCoffeeScript
+    })

src/task.coffee

@@ -49,10 +49,6 @@ class Task
     bootstrap = """
       #{coffeeScriptRequire}
       #{coffeeCacheRequire}
-      Object.defineProperty(require.extensions, '.coffee', {
-        writable: false,
-        value: require.extensions['.coffee']
-      });
       #{taskBootstrapRequire}
     """