Fixed an issue where exponential_sleep would choke on a nil. For some…

… reason, @runtime might be nil and you cannot += a nil.  It wasn't being used anywhere else anyway.

lib/cash/lock.rb

@@ -39,7 +39,7 @@ def release_lock(key)
     end
 
     def exponential_sleep(count)
-      @runtime += Benchmark::measure { sleep((2**count) / 2.0) }
+      sleep((2**count) / 2.0)
     end
 
     private