Doesn't work without this patch

jirafs crashes for me. Basically there's no trailing space after the git version.. not sure why there would be? I'm using ZSH FWIW and git version returns

```
(g) mriehl@machine ~ $ git --version
git version 1.9.1
```

This is the error I get:

```
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
/data/home/mriehl/.virtualenvs/g/bin/jirafs in <module>()
      7 if __name__ == '__main__':
      8     sys.exit(
----> 9         load_entry_point('jirafs==0.9.9', 'console_scripts', 'jirafs')()
     10     )

/data/home/mriehl/.virtualenvs/g/local/lib/python2.7/site-packages/jirafs/cmdline.pyc in main()
    490             "python before using Jirafs."
    491         )
--> 492     if utils.get_git_version() < NormalizedVersion('1.8'):
    493         raise RuntimeError(
    494             "Jirafs requires minimally version 1.8 of Git.  Please "

/data/home/mriehl/.virtualenvs/g/local/lib/python2.7/site-packages/jirafs/utils.pyc in get_git_version()
    191         stderr=subprocess.PIPE,
    192     ).decode('utf8')
--> 193     version_string = re.match('git version ([0-9.]+) .*', result).group(1)
    194     return NormalizedVersion(version_string)
    195 

AttributeError: 'NoneType' object has no attribute 'group'
```

a540cd3 fixes the issue for me.