Bump environs from 4.2.0 to 5.1.0 in /{{cookiecutter.app_name}} (#518)

Bumps [environs](https://github.com/sloria/environs) from 4.2.0 to 5.1.0.
- [Release notes](https://github.com/sloria/environs/releases)
- [Changelog](https://github.com/sloria/environs/blob/master/CHANGELOG.md)
- [Commits](https://github.com/sloria/environs/compare/4.2.0...5.1.0

)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>