Clearer output from make check in Travis

This removes the parallel make (-j) option when running make check under
Travis CI.  This will slow down the build somewhat, but makes it easier to
match errors to the modules which caused them.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

.travis.yml

@@ -16,4 +16,4 @@ addons:
 script:
         - make config.h
         - make -j2
-        - make -j2 -k check
+        - make -k check