Update rspec requirement from ~> 3.1.0 to ~> 3.8.0
Update rspec requirement from ~> 3.1.0 to ~> 3.8.0

Updates the requirements on [rspec](https://github.com/rspec/rspec) to permit the latest version.
- [Release notes](https://github.com/rspec/rspec/releases)
- [Commits](https://github.com/rspec/rspec/compare/v3.1.0...v3.8.0)
mashiro/enver