Travis: do make in parallel to test if that gives slight speedup.

Seems to.  Numbers are noisy, but before was 5 min 32 sec, after this
commit was 3 min 42 sec.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

.travis.yml

@@ -7,5 +7,5 @@ addons:
             - libjudy-dev libvorbis-dev libportaudio-dev libtalloc-dev
 
 script:
-        - make
-        - make check
+        - make -j2
+        - make -j2 check