Bump src/stager

Bump cloudfoundry-incubator/stager:
  Abhijit Hiremagalur:
     Courier execution metadata from tailor to CC ( Travis started: https://travis-ci.org/cloudfoundry-incubator/stager/builds/34860182 )
  Ted Young:
     upload and download in parallel ( Travis Success: https://travis-ci.org/cloudfoundry-incubator/stager/builds/34858032 )
  Alex Suraci:
     add concourse config ( Travis Unknown )
  Amit Gupta:
     Lock etcd in scripts/test ( Travis Success: https://travis-ci.org/cloudfoundry-incubator/stager/builds/34769836 )