Test with PHP 8.0

[phil-davis]

1. sort out variables in .travis.yml for selecting to run php-cs-fixer, phpunit and phpstan
2. Add a matrix entry to run just phpunit on Ubuntu 20.04 "focal" with "nightly" PHP (which is the way to get PHP 8.0 at the moment)
3. When not running php-cs-fixer, explicitly remove it from composer.json (because php-cs-fixer does not yet support PHP 8.0, so composer cannot sort out dependencies if we have PHP 8.0 and php-cs-fixer 2.* together)
4. Fix the constructor declarations that have an optional parameter before a required parameter (PHP 8.0 complains about these)

[codecov]

Codecov Report

Merging #160 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##             master     #160   +/-   ##
=========================================
  Coverage     93.67%   93.67%           
  Complexity      259      259           
=========================================
  Files            15       15           
  Lines           838      838           
=========================================
  Hits            785      785           
  Misses           53       53           

Impacted Files	Coverage Δ	Complexity Δ
lib/Auth/AbstractAuth.php	100.00% <100.00%> (ø)	2.00 <1.00> (ø)
lib/Auth/Digest.php	96.42% <100.00%> (ø)	20.00 <1.00> (ø)

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[phil-davis]

PR #154 started this off. Thanks @cedric-anne
I managed to sort out a way to make unit tests on PHP 8.0 run and pass.

[cedric-anne]

👍

[staabm]

Looks great, thx