Use apt-cacher-ng to speed up iteration

Signed-off-by: Matthew Boedicker <mboedicker@pivotallabs.com>
Add foodcritic to bosh-stemcell development dependencies.

Signed-off-by: Matthew Boedicker <mboedicker@pivotallabs.com>
vendor foodcritic

Signed-off-by: Jesse Zhang <jz@pivotallabs.com>