Update php version requirement

I have changed required PHP version for Elasticsearch-PHP 6.0

Author: mjavorka

docs/php-version-requirement.asciidoc

@@ -1,4 +1,4 @@
 == PHP Version Requirement
 
-Version 5.0 of Elasticsearch-PHP requires PHP version 5.6.6 or higher. In addition, it requires the native JSON
+Version 6.0 of Elasticsearch-PHP requires PHP version 7.0.0 or higher. In addition, it requires the native JSON
 extension to be version 1.3.7 or higher.