Adds nestle to the requirements

nestle is used in the tests, so is required for CI. Its lightweight and
pip installable so doesn't add too much overhead.
3 jobs for master in 3 minutes and 3 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #55072
exitcode-jessie

00:02:49

 
  Deploy
passed #55073
pages

00:00:13

passed pages:deploy

00:00:00