disable generating documentation for travis
disable generating documentation for travis

doxygen and graphviz causes travis vm to download ~400 MB from internet
and causes travis builds to timeouts occasionally.

Signed-off-by: S.Çağlar Onur <acf5ae661bb0a9f738c88a741b1d35ac69ab5408@10ur.org>
Acked-by: Stéphane Graber <089afc6d81f66f1168a9849e15660feae286e024@ubuntu.com>

QingweiPeterLan/cs188-lxc,armcc/lxc,codebauss/lxc,raspberrypython/lxc,rldleblanc/lxc,armcc/lxc,cjwatson/lxc,QingweiPeterLan/cs188-lxc,jirislaby/lxc,thmo/lxc,christiaan/lxc,hfuCN/lxc,major/lxc,ss1h2a3tw/lxc,terceiro/lxc,ksperis/lxc,ksperis/lxc,lazy404/lxc,LynxChaus/lxc,armcc/lxc,christiaan/lxc,Distrotech/lxc,ojkastl/lxc_Patches,yubo/lxc,raspberrypython/lxc,ysbnim/lxc,Ponce/lxc,JohnPeacockMessageSystems/lxc,Ponce/lxc,major/lxc,tych0/lxc,DarknessBeforeDawn/lxc,federicobriata/lxc,QingweiPeterLan/cs188-lxc,caglar10ur/lxc,caglar10ur/lxc,sapun/lxc,sapun/lxc,hkjolhede/lxc,major/lxc,JohnPeacockMessageSystems/lxc,fwilson42/lxc,Pelagicore/lxc,rowhit/lxc,GreatFruitOmsk/lxc,hfuCN/lxc,jirislaby/lxc,federicobriata/lxc,ioops/lxc,rldleblanc/lxc,szaszg/lxc,weizhenwei/lxc,hallyn/lxc,ecarrara/lxc,bmoar/lxc,fwilson42/lxc,cjwatson/lxc,raspberrypython/lxc,lisongmin/lxc,thmo/lxc,aeris/lxc,tych0/lxc,lazy404/lxc,ecarrara/lxc,yubo/lxc,hfuCN/lxc,bmoar/lxc,akshaykarle/lxc,szaszg/lxc,virajs/lxc,JohnPeacockMessageSystems/lxc,LynxChaus/lxc,cjwatson/lxc,efiop/lxc,ss1h2a3tw/lxc,ioops/lxc,efiop/lxc,Ponce/lxc,bostjan/lxc,aeris/lxc,akshaykarle/lxc,szaszg/lxc,lazy404/lxc,tych0/lxc,Distrotech/lxc,DarknessBeforeDawn/lxc,akshaykarle/lxc,rowhit/lxc,codebauss/lxc,yubo/lxc,weizhenwei/lxc,ojkastl/lxc_Patches,bmoar/lxc,GreatFruitOmsk/lxc,christiaan/lxc,efiop/lxc,ksperis/lxc,rowhit/lxc,Distrotech/lxc,jirislaby/lxc,regit/lxc,ss1h2a3tw/lxc,lisongmin/lxc,bostjan/lxc,bostjan/lxc,fwilson42/lxc,regit/lxc,hallyn/lxc,caglar10ur/lxc,virajs/lxc,ysbnim/lxc,ecarrara/lxc,lisongmin/lxc,ojkastl/lxc_Patches,thmo/lxc,weizhenwei/lxc,DarknessBeforeDawn/lxc,ioops/lxc,terceiro/lxc,Pelagicore/lxc,codebauss/lxc,aeris/lxc,federicobriata/lxc,sapun/lxc,hkjolhede/lxc,terceiro/lxc,LynxChaus/lxc,virajs/lxc,GreatFruitOmsk/lxc,rldleblanc/lxc,ojkastl/lxc_Patches,hkjolhede/lxc,hallyn/lxc,regit/lxc,Pelagicore/lxc,ysbnim/lxc