try to speed up installing lxml during travis tests
try to speed up installing lxml during travis tests

ttyS15/pybbm,jonsimington/pybbm,hovel/pybbm,webu/pybbm,artfinder/pybbm,just-work/pybbm,skolsuper/pybbm,NEERAJIITKGP/pybbm,onecue/pybbm,skolsuper/pybbm,katsko/pybbm,DylannCordel/pybbm,onecue/pybbm,wengole/pybbm,webu/pybbm,hovel/pybbm,hovel/pybbm,wengole/pybbm,skolsuper/pybbm,DylannCordel/pybbm,springmerchant/pybbm,jonsimington/pybbm,katsko/pybbm,NEERAJIITKGP/pybbm,webu/pybbm,just-work/pybbm,ttyS15/pybbm,zekone/dj_pybb,zekone/dj_pybb,NEERAJIITKGP/pybbm,springmerchant/pybbm,springmerchant/pybbm,zekone/dj_pybb,wengole/pybbm,katsko/pybbm,just-work/pybbm,ttyS15/pybbm,DylannCordel/pybbm,jonsimington/pybbm,artfinder/pybbm,artfinder/pybbm,onecue/pybbm