Travis: install lxml without compile optimisations

Use CFLAGS="-O0" as recommended by
http://lxml.de/installation.html#installation to speedup installation
times in Travis builds.

This is applied to all installations here, should maybe be limited to
lxml.

The next step wpuld be to create precompiled wheel packages to speedup
setup.

Currently we're taking about 3min to setup the packages for testing.