travis CI: no need to install libxml2 anymore
travis CI: no need to install libxml2 anymore

Fixup 6f4ff8f9191e4396530e72f933ab3acbb2965a1d

sathieu/stdeb,astraw/stdeb,LeMeteore/stdeb,astraw/stdeb,pombredanne/stdeb,sathieu/stdeb,sathieu/stdeb,pombredanne/stdeb,LeMeteore/stdeb,benthomasson/stdeb,benthomasson/stdeb,pombredanne/stdeb,astraw/stdeb,benthomasson/stdeb,LeMeteore/stdeb