add mutable flag for mkdir cache dir

For this issue(https://github.com/cloudfoundry/java-buildpack/issues/54),

There are two modifies:
1. JavaBuildpack::Util::Cache::CachedFile#initialize
Add mutable flag, whether to mkdir cache_root,defaults to true.

2. JavaBuildpack:Util::Cache::DownloadCache#from_immutable_caches
Set mutable as false, so it don't mkdir immutable_caches, in case the error of createing a directory in read-only filesystem.