Shell out when releasing.

Using this with [dokuen](https://github.com/peterkeen/dokuen), the original implementation was having problems with symlinks for some reason:

```
FileUtils.cp_r '/tmp/d20130602-29353-1mzi12u', 'releases/20130602T173351', :preserve => true
Errno::ENOENT: No such file or directory - releases/20130602T173351/bin/rake
	from /usr/local/rbenv/versions/1.9.3-p392/lib/ruby/1.9.1/fileutils.rb:1379:in `utime'
	from /usr/local/rbenv/versions/1.9.3-p392/lib/ruby/1.9.1/fileutils.rb:1379:in `copy_metadata'
	from /usr/local/rbenv/versions/1.9.3-p392/lib/ruby/1.9.1/fileutils.rb:466:in `block in copy_entry'
	from /usr/local/rbenv/versions/1.9.3-p392/lib/ruby/1.9.1/fileutils.rb:1433:in `preorder_traverse'
	from /usr/local/rbenv/versions/1.9.3-p392/lib/ruby/1.9.1/fileutils.rb:462:in `copy_entry'
	from /usr/local/rbenv/versions/1.9.3-p392/lib/ruby/1.9.1/fileutils.rb:437:in `block in cp_r'
	from /usr/local/rbenv/versions/1.9.3-p392/lib/ruby/1.9.1/fileutils.rb:1515:in `block in fu_each_src_dest'
	from /usr/local/rbenv/versions/1.9.3-p392/lib/ruby/1.9.1/fileutils.rb:1531:in `fu_each_src_dest0'
	from /usr/local/rbenv/versions/1.9.3-p392/lib/ruby/1.9.1/fileutils.rb:1513:in `fu_each_src_dest'
	from /usr/local/rbenv/versions/1.9.3-p392/lib/ruby/1.9.1/fileutils.rb:436:in `cp_r'
	from (irb):2
	from /usr/local/rbenv/versions/1.9.3-p392/bin/irb:12:in `<main>'
```

Shelling out seems to work much better in this case.