Update travis to use a newer SDCC

Due to SDCC bug #1986, we were previously stuck with SDCC revision #7100. The bug was fixed with rev #8719. As a result, we can now build contiki for 8051 ports with newer versions of the SDCC toolchain. 

This pull request updates travis to use a newer SDCC bundle.

More info: http://sourceforge.net/p/sdcc/bugs/1986/