Pull `podModulePrefix` from config/environment.js.

This brings using a custom pod prefix inline with the normal `modulePrefix`.  If not defined (by default it isn't) then it will just fallback to `modulePrefix` (this done
[here](https://github.com/stefanpenner/ember-jj-abrams-resolver/blob/master/packages/ember-resolver/lib/core.js#L171)).

Related: #1994.