added python2 exception

Author: Christopher B. Choy

demo.py

@@ -6,6 +6,9 @@
 '''
 import os
 import sys
+if (sys.version_info < (3, 0)):
+    raise Exception("Please follow the installation instruction on 'https://github.com/chrischoy/3D-R2N2'")
+
 import shutil
 import numpy as np
 from subprocess import call

main.py

@@ -1,4 +1,8 @@
 #!/usr/bin/env python
+import sys
+if (sys.version_info < (3, 0)):
+    raise Exception("Please follow the installation instruction on 'https://github.com/chrischoy/3D-R2N2'")
+
 import numpy as np
 import argparse
 import pprint