Add Slim 4 Support

Total issues resolved: **6**

- [12: Removed dependency on Pimple's ServiceProviderInterface and register](slimphp#12)
- [13: Remove PHP 5.4 from .travis.yml](slimphp#13)
- [14: Use stable version of Slim 3](slimphp#14)
- [18: Test with PHP 7.1 but allow build failures](slimphp#18)
- [21: Use no-cache header when max-age is zero](slimphp#21)
- [23: Force PHPUnit 4.0 for testing](slimphp#23)

Merge pull request slimphp#9 from dhoffend/must-revalidate

Add must-revalidate flag to Cache-Control

Bump PSR7 version requirement

Version 0.2.0

Version 0.1.0