Fix removal of symlinks to directories

When a file is a symlink to a directory, File.directory?(file) returns true, and the script used to try to delete it with Dir.delete( ) which would fail and be rescued silently as if it were a non-empty directory.
A symlink needs to be tested with File.symlink?( ) and deleted with File.delete( ).
This resolves [issue#37](https://github.com/tpkg/client/issues/37)