Fix missing VERSION file when installing via setup.py

[Lnk2past]

Fixes #92

• Moves the VERSION file into the dowhy package
• Updates the path to this file in setup.py, dowhy/__init__.py, and MANIFEST.in
• Update setup.py to include the VERSION file in the package

I initially was going to rename the VERSION file to __version__ and include that way (therefore you can just import it), but I figured this might not be what others want, so I figured I would keep things as close to the original intent as I could.

[msftclas]

All CLA requirements met.

[Lnk2past]

To be honest I am not sure if there even needs to be an entry in MANIFEST.in for VERSION, would be good if someone could confirm if that is needed at this point.

[amit-sharma]

Thanks @Lnk2past This is excellent! Yeah, I think it's probably not needed now in MANIFEST, but let's keep it like this to verify if the issue is resolved.