.travis.yml: Add clang builds to Travis

Still not exactly a great coverage of different C compilers, but clearly
better than *just* gcc.
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

.travis.yml

 language: c
 sudo: false
 
+compiler:
+  - gcc
+  - clang
+
 addons:
     apt:
         packages: