Use clang 4.0.0, not 4.0.1, since the later doesn't have an ubuntu package.

Author: emilio

ci/before_install.sh

@@ -26,7 +26,7 @@ function llvm_version_triple() {
     elif [ "$1" == "3.9" ]; then
         echo "3.9.0"
     elif [ "$1" == "4.0" ]; then
-        echo "4.0.1"
+        echo "4.0.0"
     elif [ "$1" == "5.0" ]; then
         echo "5.0.0"
     fi