Upgrade ffmpeg from 3.1.3 to 4.0

[brian-mann]

Will possibly help fix:

• Video recording works on version 1.4.2, but doesn't in versions 2.0.4 and 2.1.0 #1507
• Build hangs on CI (Traivs) #1333
• Cypress and Travis CI build fail sometimes... "No output has been received in the last 10m0s" #1934
• Blank video recording #1304

[brian-mann]

Released in 3.0.2.