Add lxml to installed packages

This should fix all of the tests in ``astropy.io.ascii`` that are failing on python 3.4.

Pinging @taldcroft and @amras1 because they were involved in the lxml-related issues that came up in astropy/astropy#2812 and astropy/astropy#2813 -- it may be that it would make sense to add lxml to the installed dependencies on the travis runs too (though the same tests that fail on windows/py34 pass on linux/py34).