Don't complain about missing module.resolve until necessary.

Calling require("reify/lib/runtime").enable(Module.prototype) before
Module.prototype.resolve has been defined is fine, as long as the resolve
method is defined before module.link calls it. If it's still undefined
then, an exception will be thrown, which fulfills the same purpose as the
exception removed by this commit.

lib/runtime/index.js

@@ -12,10 +12,6 @@ var Entry = require("./entry.js");
 // specific module objects, or for Module.prototype (where implemented),
 // to make the runtime available throughout the entire module system.
 exports.enable = function (mod) {
-  if (typeof mod.resolve !== "function") {
-    throw new Error("module.resolve not implemented");
-  }
-
   if (mod.link !== moduleLink) {
     mod.link = moduleLink;
     mod["export"] = moduleExport;