Update readme for 5.0 compatibility

README.md

@@ -2,7 +2,7 @@
 **Contributors:** automattic, ethitter  
 **Tags:** cron, cron control, concurrency, parallel, async  
 **Requires at least:** 4.4  
-**Tested up to:** 4.9  
+**Tested up to:** 5.0  
 **Requires PHP:** 7.0  
 **Stable tag:** 2.0  
 **License:** GPLv2 or later