Change Mutant::Killer#run_with_benchmark to use Benchmark.measure

* Use the real time temporarily to be consistent with the other
  places in the code that measure time. When those other areas can
  be wrapped in a Benchmark.measure block all the benchmarks will be
  updated to use cpu time, not real (elapsed) time like now.
Merge pull request #55 from mbj/use-benchmark

Change Mutant::Killer#run_with_benchmark to use Benchmark.measure