Polycom VVX x50s require firmware versions later than 5.8.x

[ivar....@onrelay.net]

Thanks for all the good work and the 19.04 update! 

We were going to test the newer Polycom x50s using 19.04, but it seems no later Polycom firmware versions than 5.5.2 are supported / listed in the 19.04 firmware dropdown menus. 

The Polycom x50 phones must have firmware version v5.8.x or later (see https://downloads.polycom.com/voice/voip/uc_sw_releases_matrix.html for a useful overview). 

Did I miss something?