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...

language: go
go:
- "1.8"
- "1.9"
- "1.10"
- "1.11"
- master
matrix:
  allow_failures:
    - go: master
  fast_finish: true
branches:
  only:
  - master
env:
  - GO111MODULE=on
install: true