Stop asserting that formatters have reference and build functions.

`reference` isn't even called directly, and there are a whole bunch of other methods that are called directly that we don't even check.

lib/container.js

@@ -21,14 +21,6 @@ function Container(options) {
     formatter,
     'missing required option `formatter`'
   );
-  assert.equal(
-    typeof formatter.reference, 'function',
-    'option `formatter` must have function `reference`'
-  );
-  assert.equal(
-    typeof formatter.build, 'function',
-    'option `formatter` must have function `build`'
-  );
 
   var resolvers = options && options.resolvers;