Fix dereferencing None when no xvfb-options given.

Running the tests without any xvfb-options was throwing.  This patch should fix it.
```
File "/usr/lib/python2.7/site-packages/nosexvfb/xvfb.py", line 26, in configure
    opts = [x.strip() for x in options.xvfb_options.split(",") if options.xvfb_options]
AttributeError: 'NoneType' object has no attribute 'split'
```