Keep working if pip isn't installed

This might be installed by invoking setup.py directly.

setup.py

@@ -4,10 +4,13 @@ import pkg_resources
 import sys
 import os
 
+try:
     if int(pkg_resources.get_distribution("pip").version.split('.')[0]) < 6:
         print('pip older than 6.0 not supported, please upgrade pip with:\n\n'
               '    pip install -U pip')
         sys.exit(-1)
+except pkg_resources.DistributionNotFound:
+    pass
 
 if os.environ.get('CONVERT_README'):
     import pypandoc