Windows 64bit build for Node v5.x fails

[Mithgol]

For the last nine days (since 79c5075, when the builds for Node.js v5.x started on AppVeyor) every Windows 64bit build for Node v5.x fails, while 32bit builds are fine.

I have no idea why.

[Mithgol]

Having identified one possible problem, I've opened an issue at appveyor/ci#497.