Fixing cache store key typo

Typo in line 19 led to cache misses and blew compilation times through the sky.
Merge pull request #434 from roblander/master

Fixing sass_cache_store issue
Note sass cache bug in changelog
Sprockets 2.9.3
Support bower.json configuration file
Note release date
Sprockets 2.10.0
Annotates exception if could be due to no .erb extension.

If an Exception is thrown during compile time and there's no erb extension
in the file name, we add a comment to the error message:

  You are using erb in your file: #{filename} but have not added a .erb
  extension.

Related notes: https://gist.github.com/schneems/5249586