Use SecureRandom.uuid for FileUtil.with_tempfile

This method that is part of the Ruby stdlib accomplishes the same thing
we had hand-rolled here, but with less effort (and likely more secure,
based on the name). See:

  http://ruby-doc.org/stdlib-2.1.0/libdoc/securerandom/rdoc/SecureRandom.html#method-c-uuid

Change-Id: I110248394897960ccfe062305ae3811034ff7ff8