[minor] Remove unneeded microtime dependency

This removes the `microtime` dependency.
It's optional and from what i saw the benchmark library builds different results and uses the one with the highest resolution. In Node.js >= 0.8 it tries to use `process.hrtime()` https://github.com/bestiejs/benchmark.js/blob/f090d93cef4604cd66de64c5dd91dfd010dc0ec5/benchmark.js#L1843-L1845 and this should be the best result.