Update travis to use mspgcc 4.7.0

This pull request updates the travis build to use mspgcc 4.7.0, which has support for 20-bit addressing allowing us to make use of the full memory of the msp430f5438a mcu that we use in the exp5438 port. The pull request also includes the latest `mspsim.jar` with support for the 20-bit msp430x instructions.

There is also a small Cooja patch that allows propagation of test script exit statuses to the exit value of the Java process, making it possible for Makefiles and other scripts to see when a test fails.