Fixed broken version checks, fixed tests.

I think this is a much more robust method of finding the default (latest) version for the nodewebkit package. 

This package has been broken since nodewebkit released new versions (a couple of hours ago), and tests have also been failing. This PR should fix things.

History:

I was getting errors when this package attempted to download invalid node-webkit binaries. The filenames had invalid semver versions like 'node-webkit-v0.10.-linux-x64.tar.gz'. I traced this down to this bit of code `v = v.slice(0, 5);`. Also, we don't want to default to the "highest version", as it may be a RC. We want to default to the latest npm version, so we use the npm api for that.