21-large-rpl: Avoid frequent timeout in Travis CI

With a timeout set to 40 min, this test often failed at about 95% of the
script, causing many spurious pull request test failures. Increase the
timeout to 50 min.

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau.dev@gmail.com>

regression-tests/21-large-rpl/testscript.js

@@ -1,5 +1,5 @@
 
-TIMEOUT(2400000); /* 40 minutes */
+TIMEOUT(3000000); /* 50 minutes */
 
 var NR_FEATHERS = mote.getSimulation().getMotesCount() - 1;