Upgrade to nan@1.2.0

:warning: __Merge this PR after atom-shell v0.14.0 gets released__ :warning: 
The PR would take a while to merge, so I was afraid we would have version conflicts when there was a new version bumped on the master before this PR was merged.

I think we can unpublish the major version and bump the minor version after the PR gets merged if bumping the major is not quite right.