Build fails with node 0.11.13 (due to microtime)

For node 0.11.13, the `npm install` step doesn't complete because microtime 0.5.1 build fails. This has been fixed in 0.6.0; the related issue is here https://github.com/wadey/node-microtime/issues/19

I tested the build after upgrading to microtime 0.6.0 and it works fine.
