travis: pass through CC to run-tests
travis: pass through CC to run-tests

fixes #2437

stapelberg/i3,Airblader/i3,Airblader/i3-original,cornerman/i3,shdown/i3,stapelberg/i3,avrelaun/i3,shdown/i3,acrisci/i3,i3/i3,cornerman/i3,i3/i3,shdown/i3,stapelberg/i3,cornerman/i3,strake/i3,strake/i3,strake/i3,acrisci/i3,Airblader/i3-original,acrisci/i3,Airblader/i3,strake/i3,shdown/i3,acrisci/i3,cornerman/i3,avrelaun/i3,stapelberg/i3,avrelaun/i3,acrisci/i3,Airblader/i3-original,Airblader/i3-original,avrelaun/i3,i3/i3,Airblader/i3,strake/i3,Airblader/i3,Airblader/i3-original,i3/i3