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...
# Created by .ignore support plugin (hsz.mobi)
### Go template
# Binaries for programs and plugins
*.exe
*.dll
*.so
*.dylib
# Test binary, build with `go test -c`
*.test
# Output of the go coverage tool, specifically when used with LiteIDE
*.out
# Project-local glide cache, RE: https://github.com/Masterminds/glide/issues/736
.glide/
*.swp
.idea/
.local/