Include association schema in primary cache key's schema hash.

@boourns & @camilo for review

## Problem

When a model has a 2 level deep denormalized has many associations, and the schema changes for the deeply associated records, then multiple associations would be reloaded after a record embedding them is fetched from the cache.  This re-introduces the problem pull request #32 tries to solve, but only occurs after a schema change without a cache flush.

## Solution

Simply include the associated schema's in the calculation of the schema hash that is used in the primary cache key prefix.