Dependency cache keys not yet generated due to absolute path to the bundle

[gesof]

I found one of the reason of issues #67 and #123 : the path to the cache folder is computed as a hash of $config object which contains an absolute path to the Bundle using SpBower. Please compute the has based on the path iself and not from the whole config object. Also consider the path starting from '%kernel.root_dir%/../' because the cli and cgi view of the path may be different (e.g. command line vs. Apache).