cmd/gopherbot: set vgo milestone automatically from subject
cmd/gopherbot: set vgo milestone automatically from subject

Change-Id: I8d893db8cee5efa5691303b4421d5940971e6a93
Reviewed-on: https://go-review.googlesource.com/95580
Reviewed-by: Andrew Bonventre <682ad9f487f614f19e8b7cc5d340bdc8e1c1b128@golang.org>

golang/build,golang/build,golang/build,golang/build