partly Revert "TravisCI: remove WFLAGS"

This reverts commit 8780800e.

.travis.yml

@@ -75,7 +75,7 @@ matrix:
               - p7zip-full
               - gcc-7
           compiler: gcc-7
-          env: GCC72=1
+          env: WFLAGS="-Wno-tautological-compare -Wno-error=implicit-fallthrough -Wno-implicit-fallthrough" GCC72=1
           #gcc-7 (Ubuntu 7.2.0-1ubuntu1~14.04) 7.2.0 20170802
         - os: linux
           addons:
@@ -90,7 +90,7 @@ matrix:
               - p7zip-full
               - gcc-8
           compiler: gcc-8
-          env: GCC81=1
+          env: WFLAGS="-Wno-tautological-compare -Wno-error=implicit-fallthrough -Wno-implicit-fallthrough -Wno-error=format-overflow" GCC81=1
           #gcc-8 (Ubuntu 7.2.0-1ubuntu1~14.04) 8.1.0
         - os: linux
           compiler: clang