kpm should not rebuild native modules unnecessarily

[skevy]

I was noticing that in subsequent kpm install runs in my project, it was rebuilding native modules even though nothing had changed. It turned out, after I put in some logging, that bcrypt and ursa were being marked as fresh every single run. As a debugging measure, I made the relevant code ignore .kpm-metadata.json: https://github.com/facebook/fbkpm/blob/master/src/util/fs.js#L214. This stopped the packages from being rebuilt. Obviously this isn't the long term solution, but I hope that helps you all diagnose and fix the real issue :)

[sebmck]

Fixed via a6d2936.