travis: bring the scons build on par with AppVeyor

Namely, always build with LLVM and run the check target.

Cc: Rhys Kidd <rhyskidd@gmail.com>
Cc: Eric Anholt <eric@anholt.net>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>

.travis.yml

@@ -109,5 +109,5 @@ script:
         ;
       make && make check;
     elif test x$BUILD = xscons; then
-      scons;
+      scons llvm=1 && scons llvm=1 check;
     fi