Add ability to log spec test order to a file

This commit tweaks the spec_helper to give it the
ability to log the spec test order to a file. It
will only log the order if you have set the
environment variable "LOG_SPEC_ORDER", e.g.:

LOG_SPEC_ORDER=true rspec spec

This will be useful when using Nick L.'s bisect
script to identify order-dependent spec failures.
It should probably be enabled on all of our
Jenkins jobs, and would potentially allow us to
start running them with the random order flag
enabled.