Add support for linux 64bit

As mentioned in https://github.com/entropi/grunt-atom-shell-app-builder/issues/8, linux-x64 wasn't supported, but it's fairly easy to add support for it.

Now the platforms `linux32` and `linux64` are available.
Support for "linux" as a platform remains for backwards compatibility. There will be a warning and automatic removal of one if there's both `linux32` and `linux` set as target platforms.