.travis.yml: Add -k to make check script

At the moment when Travis runs make check it will stop on the first
failure.  That's not particularly useful, so add a -k so that all ccanlint
failures can be seen.
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

.travis.yml

@@ -12,4 +12,4 @@ addons:
 
 script:
         - make -j2
-        - make -j2 check
+        - make -j2 -k check