Enhance patch-rn boot task to work with Windows PowerShell

[justindbelanger]

Right now, the patch-rn boot task depends upon bash. In a Windows development environment, running the patch-rn task in PowerShell will fail with the error message Cannot run program "bash": CreateProcess error=2, The system cannot find the file specified, because of this dependency. I'd love to be able to use this task on a Windows dev machine!

[pesterhazy]

Unfortunately I don't have time to update boot-react-native anymore. Re-natal works much better than boot-react-native these days and is actively maintained, so my suggestion would be to look into that.