Upgrade preferred Node version to 0.10.x

- Updates `package.json` to specify the 0.10.x engine
- No issues were found [migrating from 0.6.x to 0.8.x](https://github.com/joyent/node/wiki/API-changes-between-v0.6-and-v0.8), according to that document.
- One issue was found [migrating from 0.8.x to 0.10.x](https://github.com/joyent/node/wiki/API-changes-between-v0.8-and-v0.10):
    - Recursive `process.nextTick` -> `setImmediate`