Remove mentions of Bignum and Fixnum. These were combined into Integer in Ruby 2.4 (#5470)

Co-authored-by: shields <[email protected]>

Author: johnnyshields

lib/mongoid/criteria/queryable/key.rb

@@ -91,7 +91,7 @@ def ==(other)
 
         # Calculate the hash code for a key.
         #
-        # @return [ Fixnum ] The hash code for the key.
+        # @return [ Integer ] The hash code for the key.
         def hash
           [name, operator, expanded].hash
         end