Update benchmark for Ruby 2.1.3

Also:

- Fixed indent in benchmark
- Benchmarks pulling missing keys are now fixed to pull existing keys
- Not compatible with RSpec 3 - updated development dependency with `< 3` but eventually specs should be updated with https://github.com/yujinakayama/transpec

And finally a suggestion - the result is interesting, I think it's nice if you add a quick note in README why hashugar is faster on ephemeral use but slower on persistent use?