Fix an outdated comment in the PHP_CodeSniffer config

Author: stevegrunwell

.phpcs.xml.dist

@@ -18,7 +18,7 @@
         <exclude-pattern>tests/*</exclude-pattern>
     </rule>
 
-    <!-- Ensure we're compatible with PHP 5.6+ -->
+    <!-- Ensure we're compatible with PHP 7.1+ -->
     <rule ref="PHPCompatibility"/>
     <config name="testVersion" value="7.1-"/>
 </ruleset>