MB-8972: cleanup ruby code to spawn casperjs a bit

Particularly we're avoiding fork and making it all work without any
additional args and from inside any directory

Change-Id: I39ccd0a2ee5eab6c247fc9e9e3827b47f9a9df90
Reviewed-on: http://review.couchbase.org/29138
Reviewed-by: Pavel Blagodov <stochmail@gmail.com>
Tested-by: Pavel Blagodov <stochmail@gmail.com>