Clear require cache for config reloads.

Wrapping in a `Function` worked for simple `config/environment.js` contents but breaks down in other scenarios that relied on the file being evaluated in "nodeland".  The prior implementation prevented custom `require`'s (to get the package name from the `package.json` for instance).

Now we just clear the `require` cache to allow re-requiring the file again.

Documentation of `require.cache`:

http://nodejs.org/api/globals.html#globals_require_cache