travis: test VPATH builds
travis: test VPATH builds

It looks like VPATH (split source and build directories) builds
are frequently broken. So let's test them on travis-ci.

Personally I use VPATH build in my deployment scripts.

Signed-off-by: Aleksandr Mezin <76c1e6be84a1d2145428706eb61f38549c50daf3@gmail.com>

hfuCN/lxc,terceiro/lxc,federicobriata/lxc,cjwatson/lxc,federicobriata/lxc,GreatFruitOmsk/lxc,thmo/lxc,ss1h2a3tw/lxc,caglar10ur/lxc,thmo/lxc,QingweiPeterLan/cs188-lxc,hallyn/lxc,aeris/lxc,caglar10ur/lxc,terceiro/lxc,caglar10ur/lxc,QingweiPeterLan/cs188-lxc,aeris/lxc,cjwatson/lxc,terceiro/lxc,federicobriata/lxc,ysbnim/lxc,LynxChaus/lxc,ss1h2a3tw/lxc,GreatFruitOmsk/lxc,QingweiPeterLan/cs188-lxc,hallyn/lxc,LynxChaus/lxc,thmo/lxc,ss1h2a3tw/lxc,ysbnim/lxc,hfuCN/lxc,GreatFruitOmsk/lxc,hallyn/lxc,LynxChaus/lxc,aeris/lxc,hfuCN/lxc,cjwatson/lxc,ysbnim/lxc