Updated GPS navdata definition for firmware version 2.4.8.

I also took this opportunity to rename GPS data fields to be
consistent with existing nomenclature (that is, degreeMag instead of
degree_mag).

The new data definitions come from
https://github.com/AutonomyLab/ardrone_autonomy/pull/95 which
supposedly came via backchannels from Parrot.

I spliced a GPS navdata option collected from a drone running 2.4.8
into test/fixtures/navdata.bin and updated the unit tests.