Travis: let cppcheck fail for new files

Change Travis configuration for cppcheck to fail on new files and keep on
warning about existing ones.

.travis.yml

@@ -48,9 +48,11 @@ script:
     - make -C ./tests/unittests test BOARD=qemu-i386
     - ./dist/tools/licenses/check.sh master
 # TODO:
-#   Remove the --error-exitcode=0` when all warnings of cppcheck have been
-#   taken care of in master.
-    - ./dist/tools/cppcheck/check.sh master --error-exitcode=0
+#   Remove all but `master` parameters to cppcheck (and remove second
+#   invocation) once all warnings of cppcheck have been taken care of
+#   in master.
+    - ./dist/tools/cppcheck/check.sh master --diff-filter=MR --error-exitcode=0
+    - ./dist/tools/cppcheck/check.sh master --diff-filter=AC
 # TODO:
 #   Remove the `AC` argument when all headers have been
 #   taken care of in master.