cpu/avr/radio/rf230bb/rf230bb.c (rf230_transmit): extend comment on tx sequence

Explain that the transmission is intentionally started before copying the
frame to the buffer.
Merge pull request #304 from frtos-wpan/rf230bb-merged

cpu/avr/radio/rf230bb/rf230bb.c (rf230_transmit): extend comment on tx s...
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. Thus, we can now use newer versions

More info: http://sourceforge.net/p/sdcc/bugs/1986/
Merge pull request #319 from g-oikonomou/travis-new-sdcc

Update travis to use a newer SDCC