Add support for FileUtils#remove_entry_secure

Hello,

This is my first pull request on github, so I hope everything is done well.
I have just added an alias named "remove_entry_secure" onto "rm".
This allow to support this method of FileUtils.
I di not try to simulate the chown and chmod behavior mentionned in ruby official doc for this method. As far as I understand FakeFS, this is not necessary to fake it.

Cheers,