Add splinter related lxml dependencies.

Although splinter has a [zope.testbrowser] extras, we can not use the extras because
it installs lxml / cssselect with versions incompatible to those pinned by Plone KGS [1].

Therefore directly declare the dependencies and let the KGSs do the version pinning job.

[1] http://dist.plone.org/release/4.3-latest/versions.cfg

setup.py

@@ -7,7 +7,9 @@
 extras_require = {
     'splinter': [
         'plone.app.testing',
-        'splinter']}
+        'splinter >= 0.5.1',
+        'lxml',
+        'cssselect']}
 
 tests_require = [
     'Acquisition<4.0a1',