travis: only install lxml when testing against that implementation
travis: only install lxml when testing against that implementation

No need to stress the Travis machines with compiling lxml if it's not
going to be used anyway.

On top of that, trying to build lxml on PyPy will outright fail,
anyway.

80vs90/libsaas,ducksboard/libsaas,livingbio/libsaas,CptLemming/libsaas