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...
# Change Log
All notable changes to this project will be documented in this file.
This project adheres to [Semantic Versioning](http://semver.org/).
## [2.4.1] - 2018-4-09
### Fixed
- Pull #71, Solves #70
- Fix Travis CI Build
- Special thanks to [Vasko Zdravevski](https://github.com/vaskoz) for the PR!
## [2.4.0] - 2017-4-10
### Added
- Pull #18, Solves #17
- Add RestError Struct for an error handling
- Special thanks to [Takahiro Ikeuchi](https://github.com/iktakahiro) for the PR!
## [2.3.1] - 2016-10-14
### Changed
- Pull #15, solves Issue #7
- Moved QueryParams processing into BuildRequestObject
- Special thanks to [Gábor Lipták](https://github.com/gliptak) for the PR!
## [2.3.0] - 2016-10-04
### Added
- Pull [#10] [Allow for custom Content-Types](https://github.com/sendgrid/rest/issues/10)
## [2.2.0] - 2016-07-28
### Added
- Pull [#9](https://github.com/sendgrid/rest/pull/9): Allow for setting a custom HTTP client
- [Here](https://github.com/sendgrid/rest/blob/master/rest_test.go#L127) is an example of usage
- This enables usage of the [sendgrid-go library](https://github.com/sendgrid/sendgrid-go) on [Google App Engine (GAE)](https://cloud.google.com/appengine/)
- Special thanks to [Chris Broadfoot](https://github.com/broady) and [Sridhar Venkatakrishnan](https://github.com/sridharv) for providing code and feedback!
## [2.1.0] - 2016-06-10
### Added
- Automatically add Content-Type: application/json when there is a request body
## [2.0.0] - 2016-06-03
### Changed
- Made the Request and Response variables non-redundant. e.g. request.RequestBody becomes request.Body
## [1.0.2] - 2016-04-07
### Added
- these changes are thanks to [deckarep](https://github.com/deckarep). Thanks!
- more updates to error naming convention
- more error handing on HTTP request
## [1.0.1] - 2016-04-07
### Added
- these changes are thanks to [deckarep](https://github.com/deckarep). Thanks!
- update error naming convention
- explicitly define supported HTTP verbs
- better error handing on HTTP request
## [1.0.0] - 2016-04-05
### Added
- We are live!