Fix INSTALL for new lxml dependency

INSTALL

@@ -5,7 +5,7 @@ Prerequisites
 -------------
 
  * Python 2.4 or later (http://www.python.org/)
- * lxml (http://codespeak.net/lxml/)
+ * lxml 2.0 or later (http://codespeak.net/lxml/)
  * relatorio 0.2.0 or later (http://relatorio.openhex.org/)
  * python-dateutil (http://labix.org/python-dateutil)
  * simplejson (http://undefined.org/python/#simplejson)