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...
// Package toml is a TOML parser and manipulation library.
//
// This version supports the specification as described in
// https://github.com/toml-lang/toml/blob/master/versions/en/toml-v0.4.0.md
//
// Marshaling
//
// Go-toml can marshal and unmarshal TOML documents from and to data
// structures.
//
// TOML document as a tree
//
// Go-toml can operate on a TOML document as a tree. Use one of the Load*
// functions to parse TOML data and obtain a Tree instance, then one of its
// methods to manipulate the tree.
//
// JSONPath-like queries
//
// The package github.com/pelletier/go-toml/query implements a system
// similar to JSONPath to quickly retrieve elements of a TOML document using a
// single expression. See the package documentation for more information.
//
package toml