travis: Use parallel build

Not sure if 4 jobs is optimal, but according to the docs each build host
has 1.5 virtual cores available (although "getconf _NPROCESSORS_ONLN"
returns 32, which is probably the number of real cores underneath), so
more jobs might not actually reduce the build time much more.

scripts/test.sh

@@ -62,4 +62,4 @@ CONFIG="$CONFIG
 	--enable-leak-detective=${LEAK_DETECTIVE-no}"
 
 echo "$ ./configure $CONFIG && make $TARGET"
-./configure $CONFIG && make $TARGET
+./configure $CONFIG && make -j4 $TARGET