Checking in changes prior to tagging of version v0.4.0

## [v0.4.0](https://github.com/Songmu/timeout/compare/v0.3.1...v0.4.0) (2019-04-21)

* introduce Go Modules [#21](https://github.com/Songmu/timeout/pull/21) ([Songmu](https://github.com/Songmu))