Use vendoring for backward compatibility The switch of the Madon library to Go modules breaks builds with Go version < 1.11 since the import path now contains "v2". This patch adds vendoring so that it can still build with those versions. Let's try to re-enable Travis builds with Go v1.8-1.11...

# This source code was written by the Go contributors.
# The master list of contributors is in the main Go distribution,
# visible at http://tip.golang.org/CONTRIBUTORS.