Remove removal of DISPLAY environment variable

The issue has to do with the two lines:

` if "DISPLAY" in os.environ:
    del os.environ["DISPLAY"]`

This seems to remove the DISPLAY environment variable unnecessarily, as on line 50 of xvfbwrapper.py, self.orig_display is set to the value of DISPLAY. self.orig_display is checked on line 83, which is where the error occurs. Because of xvfb.py removing the environment variable and self.orig_display being set to the original value, on line 84 when it tries to remove DISPLAY, it has already been removed by xvfb.py, so it throws a KeyError.

dryscrape/xvfb.py

@@ -3,15 +3,15 @@
 
 _xvfb = None
 
+
 def start_xvfb():
-  from xvfbwrapper import Xvfb
-  global _xvfb
-  if "DISPLAY" in os.environ:
-    del os.environ["DISPLAY"]
-  _xvfb = Xvfb()
-  _xvfb.start()
-  atexit.register(_xvfb.stop)
+    from xvfbwrapper import Xvfb
+    global _xvfb
+    _xvfb = Xvfb()
+    _xvfb.start()
+    atexit.register(_xvfb.stop)
+
 
 def stop_xvfb():
-  global _xvfb
-  _xvfb.stop()
+    global _xvfb
+    _xvfb.stop()