Update NMEA refs $PGTOP to $PCD for new modules

[patrickslarson8]

Scope:

• Changes NMEA_parse.cpp and Adafruit_GPS.h. 3 lines total, renaming an old
  value to a new one.

Known Limitations:

• Will break compatability with GPS modules before v3.1.

Fixes #148

[wolfmanjm]

FYI You also need to modify the following two lines in Adafruit_PMTK.h....

#define PGCMD_ANTENNA   "$CDCMD,33,1*7C" ///< request for updates on antenna status
#define PGCMD_NOANTENNA "$CDCMD,33,0*7D" ///< don't show antenna status messages