Adds XCache and APC to the "sugggest" key

Either XCache or APC is required, so it's not possible to add either to
the 'require' key. Adding both to the suggest key is an attempt to alert
users who didn't read the docs closely enough that they should install
one or the other.
Updates suggest text.
Merge pull request #12 from jeremykendall/feature/update-composer

Adds "suggest" key to suggest either XCache or APC