Fix CI.

Author: paurkedal

.travis.sh

@@ -1,6 +1,7 @@
 #! /bin/sh
 set -ex
 cd `dirname $0`
+sudo apt -y install m4
 opam pin add -yn ${PKG_NAME} .
 opam depext -y ${PKG_NAME}
 opam install -yt ${PKG_NAME}