Check for existance of git-python before checking it's version attribute

Otherwise, we get these unfriendly stack traces:
[ERROR   ] Failed to read the virtual function for fileserver: gitfs
Traceback (most recent call last):
  File "/usr/lib/pymodules/python2.7/salt/loader.py", line 686, in gen_functions
    virtual = mod.__virtual__()
  File "/usr/lib/pymodules/python2.7/salt/fileserver/gitfs.py", line 41, in __virtual__
    if not git.__version__ > '0.3.0':
NameError: global name 'git' is not defined