Update request module and node engine requirements

As mentioned in #887, the current `request` module (v2.14.0) that's used in YUI doesn't work well with how Node v0.10.0 handles HTTPS CAs, since the default SSL mode in v0.10.0 is now strict.

This PR updates the `request` module to the latest version (v2.21.0), which [fixes this issue](https://twitter.com/wangbus/status/312327957718913028), and also adjusts the Node engine requirement to be at least v0.8.0, which is the minimum Node engine required for the latest version of `request`.  This is also the minimum version of Node that's supported according to [YUI Target Environments](http://yuilibrary.com/yui/environments/), so it shouldn't be a problem.