add focal to travis, and debian stretch / buster to Github action to catch errors like Error:SSL routines:SSL_CTX_use_certificate:ee key too small

[MaxPeal]

to catch bugs like Error:SSL routines:SSL_CTX_use_certificate:ee key too small

https://wiki.debian.org/ContinuousIntegration/TriagingTips/openssl-1.1.1

[coveralls]

Coverage remained the same at 100.0% when pulling d651a62 on MaxPeal:patch-1 into a0309e6 on Rob--W:master.

[Rob--W]

What errors did you catch by enabling this option? The tests are supposedly independent of the underlying TLS library.

[MaxPeal]

please see the error of the github action,
the tests are not independent of the underlying TLS library.
on debian stretch its tests ok,
https://github.com/MaxPeal/cors-anywhere/runs/1099395503?check_suite_focus=true
https://github.com/MaxPeal/cors-anywhere/actions?query=workflow%3ADocker-Image-CI-stretch

on debian buster it FAIL
https://github.com/MaxPeal/cors-anywhere/actions/runs/248959691
https://github.com/MaxPeal/cors-anywhere/actions?query=workflow%3ADocker-Image-CI-buster

my PR with out github action dont work, PR for RfC #273

[MaxPeal]

the ERROR MSG:
100 passing (5s)
2 failing

1. server on https "before all" hook:
   Error: error:140AB18F:SSL routines:SSL_CTX_use_certificate:ee key too small
   at Object.createSecureContext (_tls_common.js:129:17)
   at Server.setSecureContext (_tls_wrap.js:1323:27)
   at Server (_tls_wrap.js:1181:8)
   at new Server (https.js:66:14)
   at Object.createServer (https.js:90:10)
   at createServer (lib/cors-anywhere.js:417:31)
   at Context. (test/test.js:500:21)
   at processImmediate (internal/timers.js:461:21)

2. server on https "after all" hook:
   TypeError: Cannot read property 'close' of null
   at stopServer (test/test.js:35:17)
   at Context. (test/test.js:517:5)
   at processImmediate (internal/timers.js:461:21)

npm ERR! code ELIFECYCLE
npm ERR! errno 2
npm ERR! cors-anywhere@0.4.3 test: mocha ./test/test*.js --reporter spec
npm ERR! Exit status 2
npm ERR!