Update squizlabs/php_codesniffer requirement from ^3.5 to ^3.6
Update squizlabs/php_codesniffer requirement from ^3.5 to ^3.6

Updates the requirements on [squizlabs/php_codesniffer](https://github.com/squizlabs/PHP_CodeSniffer) to permit the latest version.
- [Release notes](https://github.com/squizlabs/PHP_CodeSniffer/releases)
- [Commits](https://github.com/squizlabs/PHP_CodeSniffer/compare/3.5.0...3.6.0)

Signed-off-by: dependabot[bot] <5bdcd3c0d4d24ae3e71b3b452a024c6324c7e4bb@github.com>
slimphp/Slim-Http