Ensure permissions are preserved when copying package

`FileUtils.cp` on Ruby 1.9 has a fun bug where permissions are not  preserved unless the `preserve` option is set to true:
https://bugs.ruby-lang.org/issues/4507

This has the nasty side affects of removing the executable bit on makeself-generated self-extracting tarballs!

/cc @opscode/release-engineers @opscode/client-engineers 

This fix will also need to be ported to the 3.0-stable branch.