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.
Merge pull request #1809 from rwjblue/properly-clear-node-cache

Clear require cache for config reloads.