travis: maybe fix cmake gcc

travis-linux

@@ -1,6 +1,11 @@
 #!/bin/bash
 set -euo pipefail
 
+echo "CC is ${CC:-unknown}"
+echo "CXX is ${CXX:-unknown}"
+
+export CC=gcc
+export CXX=g++
 sudo apt-get install software-properties-common
 sudo add-apt-repository --yes ppa:george-edison55/cmake-3.x
 sudo apt-get update