Fix wrong #exist? call
Use #rm_rf rather than #remove_entry_secure

On some systems, `FileUtils#remove_entry_secure` fails to work because
it fails the [TOCTTOU vulnerability check](1), even for `/tmp`. In the
case of nanoc, the vulnerability check has little use, since it deletes
the directories that it creates itself, so using `FileUtils#rm_rf`
instead of `FileUtils#remove_entry_secure` is fine.

Also see nanoc/nanoc#465

[1]: http://www.ruby-doc.org/stdlib-2.1.2/libdoc/fileutils/rdoc/FileUtils.html#method-c-remove_entry_secure