Don't define Array#sum if already defined. Fixes #58

Recent versions of ActiveSupport and Ruby 2.4.1 have added a #sum method
to the Enumerable class. Their implementation also accepts a block,
which the version included with StatSample does not.

This change tests to see if Array#sum is already defined before
patching.

This fixes issue #58 #58

lib/statsample.rb

@@ -54,8 +54,10 @@ def include_aliasing(m, suffix = 'ruby')
 end
 
 class Array
-  def sum
-    inject(:+)
+  unless method_defined?(:sum)
+    def sum
+      inject(:+)
+    end
   end
 
   def mean