title: easyprocess.EasyProcessCheckInstalledError: cmd=['Xvfb', '-help'] OSError=[Errno 2] No such file or directory
body: <p>I'm trying to setup a display but it's telling me <code>No such file or directory</code>.</p>
  
  <pre><code>Traceback (most recent call last):
    File "/var/www/envs/venv/proj/scripts/my_file.py", line 44, in click
      display = Display(visible=0, size=(800,600))
    File "/usr/lib/python2.6/site-packages/pyvirtualdisplay/display.py", line 33, in __init__
      self._obj = self.display_class(
    File "/usr/lib/python2.6/site-packages/pyvirtualdisplay/display.py", line 51, in display_class
      cls.check_installed()
    File "/usr/lib/python2.6/site-packages/pyvirtualdisplay/xvfb.py", line 38, in check_installed
      ubuntu_package=PACKAGE).check_installed()
    File "/usr/lib/python2.6/site-packages/easyprocess/__init__.py", line 209, in check_installed
      raise EasyProcessCheckInstalledError(self)
  easyprocess.EasyProcessCheckInstalledError: cmd=['Xvfb', '-help']
  OSError=[Errno 2] No such file or directory
  Program install error!
  </code></pre>