Revert 31349577's change on lxml version pinning.

We should not pin on alpha versions. And this one doesn't exist anymore on
pypi.

component/slapos/buildout.cfg

@@ -124,7 +124,7 @@ Werkzeug = 0.8.3
 buildout-versions = 1.7
 collective.recipe.template = 1.9
 hexagonit.recipe.cmmi = 1.6
-lxml = 3.0alpha2
+lxml = 2.3.5
 meld3 = 0.6.9
 netaddr = 0.7.10
 slapos.core = 0.29