travis: Add Clang as compiler

Travis also supports clang as a compiler, so we should use it as another
checking instance for our code - and maybe even the static code analyzer

Signed-off-by: Peter Huewe <peterhuewe@gmx.de>
diff --git a/.travis.yml b/.travis.yml
index 3944ac1..18bbb4e 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,6 +1,7 @@
 language: cpp
 compiler:
   - gcc
+  - clang
 
 # This is a lie: we don't need sudo but this is required to get an
 # Ubuntu image with a libc that isn't ancient, and with cmocka libs.