splinter is no longer required to be installed from source.
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