avoid b' in get_datafile

[graingert]

Summary

• get_datafile feature is limited to Python 2. When Py 2 is used to fetch the datafile using get_datafile() the returned datafile is unicode string.
• When Py 3 is used, the get_datafile() method returns 'bytes' type, a binary string), not a regular text. This requires extra encoding to convert. The patchmakes datafile json string compatible with both Py2 and Py3.
• ProjectConfig's init sets self._datafile = u'{}'.format(datafile) but the next line does self._datafile = u'{}'.format(datafile)
• [PS: bytes/unicode mixups happen a lot when porting py2 code to py3]

Test plan

• fix to be tested in both Py2 and Py3.

Issues

• “THING-1234” or “Fixes Overhaul logging integration. #123”

[andrewleap-optimizely]

fixed by #384