Use null, not '*', for unspecified version numbers

[benwaffle]

No description provided.

[romgrk]

Will test this tonight. In an ideal world, there would be tests for everything, but it's not essential.
Mocha is already set up (see example https://github.com/romgrk/node-gtk/blob/master/test/function_call.js). Also C++ functions are exposed to JS through require('node-gtk').c, so it's actually fun to write tests :)