try a different way of checking for expected content

* the current way is stalling ~50% of the time when run with babushka
  0.26.9 and ruby2.0.0p195
* The curl command is completing but strace shows it stalls on writing
  to STDOUT
* The ruby process is stalled on reading from curl's STDOUT
* I don't really have time to track down if the issue relates to ruby2,
  babs or the combination, so this is an attempt to perform the check
  using grep and only send a single character over STDOUT to see if that
  helps