Travis: still need to keep -Wno-tautological-compare for GCC 7

.travis.yml

@@ -113,7 +113,7 @@ matrix:
               - p7zip-full
               - gcc-7
           compiler: gcc-7
-          env: WFLAGS="-Wno-error=implicit-fallthrough"
+          env: WFLAGS="-Wno-tautological-compare -Wno-error=implicit-fallthrough"
           #gcc-7 (Ubuntu 7.2.0-1ubuntu1~14.04) 7.2.0 20170802
         - os: linux
           compiler: clang