README.md: fix Travis link
# Contiki-NG: the Next Generation Contiki
-Travis link: https://travis-ci.com/contiki-ng/contiki-ng
+Travis link: https://travis-ci.org/contiki-ng/contiki-ng
Contiki-NG is a fork of Contiki, with focus on next generation IoT protocols and platforms: http://contiki-ng.org
