slugrunner: Start slugrunner jobs as the nobody user

Signed-off-by: Lewis Marshall <lewis@lmars.net>
slugbuilder: Fix whitespace

Signed-off-by: Lewis Marshall <lewis@lmars.net>
slugbuilder: Run buildpacks as the nobody user

Signed-off-by: Lewis Marshall <lewis@lmars.net>
Merge pull request #360 from flynn/run-apps-as-nobody

Build and run apps as the nobody user