[stats] Fix key search in dictionary

beat/core/stats.py

@@ -233,7 +233,7 @@ def cpu_statistics(start, end):
 
     """
 
-    if not end.has_key('system_cpu_usage'):
+    if 'system_cpu_usage' not in end:
         return {
                 'user': 0.0,
                 'system': 0.0,