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 has been changed and the
HTTP response is a 301 redirect. curl doesn't follow it

We change .travis.yml to pull the bundles form the new location
Merge pull request #662 from g-oikonomou/travis-url-fix

Fix Travis failure
llsec: Added a layer in between NETSTACK_MAC and NETSTACK_NETWORK for implementing link layer security
llsec: Configuration option for defining a constant LoWPAN-wide security level
Sky: Calling NETSTACK_LLSEC.bootstrap from contiki-sky-main.c
Sky: DEBUG on/off switch
packetbuf: Made packetbuf_hdrlen applicable to inbound packets, too
framer-802154: Style fixes
framer-802154: Introduced mnemonic variable
framer-802154: Support for creating and parsing frames with security headers
framer-802154: Support for setting and accessing the frame type
attribute
framer-802154: Sequence number fix
framer-802154: Renamed variable for the sake of consistency
framer-802154: Simplification of frame802154_create
framer-802154: Conditional compilation of security-related code
framer-802154: Support for explicit keys
llsec: Calling NETSTACK_LLSEC.on_frame_created()
llsec: Allow for sharing AES-128
llsec: Allow for sharing CCM*
llsec: CCM* verification tests