The check-format travis test might not work with clang5.0
so stay with clang 4 and hence the deprecated trusty image of travis.
We will have to find a better solution eventually
diff --git a/.travis.yml b/.travis.yml
index eed23e9..c169310 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -5,6 +5,11 @@
sudo: required
dist: trusty
+# the check-format test seems to fail with clang-format 5.0, so for
+# now stay with the older version of clang and hence the whole trusty
+# image
+group: deprecated-2017Q4
+
language: c
services: