Travis: Switch to Ubuntu 20.04 Focal for FFmpeg 4 build

[ferdnyc]

Travis appear to have quietly brought up Ubuntu Focal as an available build dist (still undocumented anywhere, AFAICT). Still, using an undocumented Travis environment is the lesser evil, vs. using FFmpeg packages from a community PPA that's already bailed on us once.

[codecov]

Codecov Report

Merging #522 into develop will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop     #522   +/-   ##
========================================
  Coverage    48.33%   48.33%           
========================================
  Files          128      128           
  Lines         9962     9962           
========================================
  Hits          4815     4815           
  Misses        5147     5147           

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[ferdnyc]

Merging this, as it allows us to eliminate all community/unofficial PPA usage (except our own!) in the CI setup, other than the FFmpeg 3.2 problem-child which remains in limbo ATM.

Travis's Focal support isn't likely to go away now that they've let that genie out of its bottle, I'm sure it'll become official soon enough.

cc: @jonoomph

[ferdnyc]

FFmpeg 3.2 will probably just end up getting dropped. Adding it as a test version in #435 was a "nice to have", after #433 revealed some issues with code that was claimed to be FFmpeg 3.2 compatible but actually required 3.4... but it was never critical. Those are both outdated versions not worth expending resources on.

When adding 3.2 was simple, it made sense. No longer simple? No longer makes sense.