Test with PHP8.0

[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) libxml_disable_entity_loader is deprecated on PHP 8.0. This is already disabled on PHP 8.0, so this function does not need to be called. Put a conditional check around it. https://php.watch/versions/8.0/libxml_disable_entity_loader-deprecation

Relates to https://github.com/sabre-io/vobject/pull/513

[codecov[bot]]

Codecov Report

Merging #190 into master will increase coverage by 0.04%. The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master     #190      +/-   ##
============================================
+ Coverage     97.61%   97.65%   +0.04%     
- Complexity      112      116       +4     
============================================
  Files            13       13              
  Lines           461      469       +8     
============================================
+ Hits            450      458       +8     
  Misses           11       11              

Impacted Files	Coverage Δ	Complexity Δ
lib/Reader.php	99.16% <100.00%> (+0.03%)	46.00 <0.00> (+3.00)
lib/Writer.php	100.00% <0.00%> (ø)	19.00% <0.00%> (ø%)
lib/Service.php	100.00% <0.00%> (ø)	18.00% <0.00%> (ø%)

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