travis: use most recent bleeding-edge-toolchain (150928) with GCC 5.3.0
travis: use most recent bleeding-edge-toolchain (150928) with GCC 5.3.0

This toolchain no longer requires any non-system shared libraries, so
there's no need to "relink" libfl.
DISTORTEC/distortos,DISTORTEC/distortos,jasmin-j/distortos,jasmin-j/distortos,jasmin-j/distortos,CezaryGapinski/distortos,CezaryGapinski/distortos,jasmin-j/distortos,jasmin-j/distortos,DISTORTEC/distortos,CezaryGapinski/distortos,CezaryGapinski/distortos,CezaryGapinski/distortos,DISTORTEC/distortos