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/clangtidy.sh

@@ -3,9 +3,7 @@
 
 needs_compile || exit 0
 
-if hash clang-tidy-5.0 2>/dev/null; then
-	CLANG_TIDY=clang-tidy-5.0
-else
+if [ -z "${CLANG_TIDY}" ]; then
 	CLANG_TIDY=clang-tidy
 fi