Remove XPath string() conversion of element in CSS "contains" function translation

[redapple]

According to http://www.w3.org/TR/xpath/#section-Function-Calls (and in practice using lxml), each argument is "converted (...) to the type required by the function,".

So, the string(.) conversion on the context node is redundant.
Very minor, and probably only cosmetic.

[SimonSapin]

Thanks for contributing. I’m ok with this change, but please:

• Change the Travis file in a separate commit
• Avoid unrelated whitespace change, or if you think they’re important do them in a separate commit.

[redapple]

Sorry for the multiple commits, I got mixed up in my reverts.
Any way to clean that up?

[redapple]

Rebased to make things cleaner.

[SimonSapin]

Merged, thanks.

[redapple]

De rien Simon !
cssselect is pretty amazing, so I'm glad I can help out (even those small things)

[SimonSapin]

Well, I’ve grown to hate XPath but that’s what we have :) (So far…)

[redapple]

Well, I'd be happy to discuss #29 to see how to properly extend cssselect translators for that extra bit "missing" in CSS ;-)