feat(Makefile): pull deis/slugrunner

Pulling deis/slugrunner on every host will significantly speed up
both application launch times on nodes that have not yet started an
app as well as speeding up launch times for deis/registry on vagrant
deploys or any other deployment option that utilizes `make pull`.
Merge pull request #832 from deis/slugrunner-pull

feat(Makefile): pull deis/slugrunner