Make the kotlin linter runnable in python 3
Make the kotlin linter runnable in python 3

Summary:

The kotlin linter would fail if run under python 3, since we were
trying to parse the java version using a regexp against bytes. We can
fix by correcly decoding to string.

Test Plan:
- with a python3 virtualenv active, run `ka-lint` on some kotlin code,
  see that there's no error

Auditors: jacqueline, benkraft

Khan/khan-linter,Khan/khan-linter,Khan/khan-linter,Khan/khan-linter