The gem resource used to determine if a gem is installed based on the exit
status of the `gem` command, however that command will return zero
if the package was found or not. This patch checks to ensure that the
`gem list` command actually includes the gem name or is empty to
determine if the gem is in fact installed.
If the gem command returns something other than a `0` exit code, then
it'll skip the resource.
Signed-off-by: Keith Walters <keith.walters@cattywamp.us>
