[travis] enable APT caching
[travis] enable APT caching

ycsoft/qxmpp,LightZam/qxmpp,gamenet/qxmpp,Kaffeine/qxmpp,gotomypc/qxmpp,gotomypc/qxmpp,LightZam/qxmpp,olesalscheider/qxmpp,olesalscheider/qxmpp,pol51/QXMPP-CMake,deepak899/qxmpp,ycsoft/qxmpp,gotomypc/qxmpp,cccco/qxmpp,cccco/qxmpp,cccco/qxmpp,Ilyatk/qxmpp,kollix/qxmpp,kollix/qxmpp,KangLin/qxmpp,gamenet/qxmpp,ycsoft/qxmpp,KangLin/qxmpp,LightZam/qxmpp,Ilyatk/qxmpp,deepak899/qxmpp,deepak899/qxmpp,Kaffeine/qxmpp,Ilyatk/qxmpp,gamenet/qxmpp,KangLin/qxmpp,olesalscheider/qxmpp,pol51/QXMPP-CMake,kollix/qxmpp,Kaffeine/qxmpp,pol51/QXMPP-CMake