Set parentdir_prefix in setup.py to fix egg version

[DarkDefender]

Hello, when trying to install this from the latest version tarball, I ran into an issue where versioneer couldn't figure out the correct version (and thus set it to 0+unknown).

This make the pyls project unable to find it.

I'm unsure if this is the correct way to go about fixing this issue. But it seems work for me.

[palantirtech]

Thanks for your interest in palantir/python-jsonrpc-server, @DarkDefender! Before we can accept your pull request, you need to sign our contributor license agreement - just visit https://cla.palantir.com/ and follow the instructions. Once you sign, I'll automatically update this pull request.