Update simplecov requirement to ~> 0.15.0
Update simplecov requirement to ~> 0.15.0

Updates requirements on [simplecov](https://github.com/colszowka/simplecov) to permit the latest version.
- [Changelog](https://github.com/colszowka/simplecov/blob/master/CHANGELOG.md)
restforce/restforce,restforce/restforce,ejholmes/restforce,ejholmes/restforce