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

.clang-format

@@ -13,6 +13,7 @@ BraceWrapping:
   AfterUnion: true
   BeforeCatch: false
   BeforeElse: false
+FixNamespaceComments: false
 AllowShortIfStatementsOnASingleLine: false
 IndentCaseLabels: false
 AccessModifierOffset: -8