Check if the app is run in python3 (#438)

rsf.py

-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 from __future__ import print_function
+import sys
+if sys.version_info.major < 3:
+    print("RouterSploit supports only Python3. Rerun application in Python3 environment.")
+    exit(0)
 
 import logging.handlers
-
 from routersploit.interpreter import RoutersploitInterpreter
 
 log_handler = logging.handlers.RotatingFileHandler(filename="routersploit.log", maxBytes=500000)