Fix Travis failure

For travis tests, we host pre-built bundles of the arm-gcc and sdcc toolchains on github, and we download them with curl.

Apparently, the URL for those bundles changed at some point, and the HTTP response is now a 301 redirect. curl doesn't follow it.

This pull changes .travis.yml to download the bundles from the new location

Reported by @cmorty (thanks) At the start I thought it was transient, but apparently it's not.

http://article.gmane.org/gmane.os.contiki.devel/22053