0.4.2

This release fixes a break which was caused by github.com/bazelbuild/buildifier being renamed to buildtools. The following changes are cherry-picked:

• #319 go_repository: add importpath and vcs flags. This allows us to have separate import paths and remote URLs for repositories.
• #362 Fetch buildifier from new location. This allows the buildifier zip to be extracted with the new name.