Support phpunit/php-code-coverage 6 (#7)

Author: kylekatarnls

composer.json

@@ -29,7 +29,7 @@
     "require": {
         "php": ">=5.5.0",
         "phpunit/phpunit": "^4.8.35 || ^5.7 || ^6.0 || ^7.0",
-        "phpunit/php-code-coverage": "^2.2 || ^4.0 || ^5.2",
+        "phpunit/php-code-coverage": "^2.2 || ^4.0 || ^5.2 || ^6.0",
         "squizlabs/php_codesniffer": "^2.8",
         "symfony/console": "^3.2",
         "codeclimate/php-test-reporter": "^0.4.0"