fix failing tests

saucebrush/stats.py

@@ -1,5 +1,4 @@
 from saucebrush.filters import Filter
-from saucebrush.utils import FallbackCounter
 import collections
 import math
 
@@ -202,10 +201,7 @@ class Histogram(StatsFilter):
 
     def __init__(self, field, **kwargs):
         super(Histogram, self).__init__(field, **kwargs)
-        if hasattr(collections, "Counter"):
-            self._counter = collections.Counter()
-        else:
-            self._counter = FallbackCounter()
+        self._counter = collections.Counter()
 
     def process_field(self, item):
         self._counter[self.prep_field(item)] += 1