Fixing pip install pygit2

Regression in the latest release, "pip install pygit2" fails if cffi is
not already installed. Should be fixed with this change, not tested.

setup.py

@@ -46,7 +46,11 @@ import sys
 import unittest
 
 # Get cffi major version
+try:
     import cffi
+except ImportError:
+    cffi_major_version = None
+else:
     cffi_major_version = cffi.__version_info__[0]
 
 # Import stuff from pygit2/_utils.py without loading the whole pygit2 package