Parallel compilations appears to troll hash-for-dep cache

I noticed that rebuild time for a particular addon (with a lot of handlebars AST transforms) recently ballooned from a few seconds to 80-90 seconds.

It appears that something (I'm guessing the parallel compile) is causing `hash-for-dep` to be invoked as multiple modules, instead of a single module. Since the `hash-for-dep` cache [is module scoped](https://github.com/stefanpenner/hash-for-dep/blob/master/index.js#L10), invoking as multiple modules means that the cache is effectively useless.

EDIT: The cache was only created once in `ember-cli-htmlbars-inline-precompile@1.0.2`. When I upgraded to `1.0.3` is when the multiple caches thing happened. There were other things also upgraded though at the same time.

cc @stefanpenner