Update our tooling (Clang 5 -> 7, GCC 7 -> 8)

This change permits to use up-to-date compilers, clang-tidy and
clang-format

It also refactor the tidy/format step to drop the binary selection from
scripts and perform it directly in travis

util/travis/lint.sh

@@ -1,9 +1,7 @@
 #! /bin/bash
 function perform_lint() {
 	echo "Performing LINT..."
-	if hash clang-format-5.0 2>/dev/null; then
-		CLANG_FORMAT=clang-format-5.0
-	else
+	if [ -z "${CLANG_FORMAT}" ]; then
 		CLANG_FORMAT=clang-format
 	fi
 	echo "LINT: Using binary $CLANG_FORMAT"