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...
module github.com/spf13/viper
require (
github.com/fsnotify/fsnotify v1.4.7
github.com/hashicorp/hcl v1.0.0
github.com/magiconair/properties v1.8.0
github.com/mitchellh/mapstructure v1.0.0
github.com/pelletier/go-toml v1.2.0
github.com/spf13/afero v1.1.2
github.com/spf13/cast v1.2.0
github.com/spf13/jwalterweatherman v1.0.0
github.com/spf13/pflag v1.0.2
golang.org/x/sys v0.0.0-20180906133057-8cf3aee42992 // indirect
golang.org/x/text v0.3.0 // indirect
gopkg.in/yaml.v2 v2.2.1
)