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. Fixup calendar parameter to Broker parseEvent

[codecov]

Codecov Report

Merging #513 into master will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master     #513      +/-   ##
============================================
- Coverage     98.69%   98.67%   -0.02%     
- Complexity     1757     1776      +19     
============================================
  Files            66       66              
  Lines          4279     4304      +25     
============================================
+ Hits           4223     4247      +24     
- Misses           56       57       +1     

Impacted Files	Coverage Δ	Complexity Δ
lib/ITip/Broker.php	99.50% <100.00%> (ø)	155.00 <21.00> (+1.00)
lib/Property/Boolean.php	50.00% <0.00%> (-3.85%)	6.00% <0.00%> (ø%)
lib/Property/UtcOffset.php	100.00% <0.00%> (ø)	3.00% <0.00%> (ø%)
lib/Property/ICalendar/Period.php	100.00% <0.00%> (ø)	9.00% <0.00%> (ø%)
lib/Property/ICalendar/DateTime.php	100.00% <0.00%> (ø)	44.00% <0.00%> (ø%)
lib/Property/VCard/DateAndOrTime.php	100.00% <0.00%> (ø)	75.00% <0.00%> (ø%)
lib/Parser/MimeDir.php	99.55% <0.00%> (+<0.01%)	85.00% <0.00%> (+3.00%)
lib/Parameter.php	99.20% <0.00%> (+<0.01%)	82.00% <0.00%> (+1.00%)
lib/Property/Text.php	99.13% <0.00%> (+<0.01%)	41.00% <0.00%> (+1.00%)
lib/FreeBusyGenerator.php	98.11% <0.00%> (+<0.01%)	90.00% <0.00%> (ø%)
... and 6 more

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[phil-davis]

2) Sabre\VObject\Parser\XmlTest::testRFC6321Example1
Function libxml_disable_entity_loader() is deprecated
/home/travis/build/sabre-io/vobject/vendor/sabre/xml/lib/Reader.php:58
/home/travis/build/sabre-io/vobject/lib/Parser/XML.php:358
/home/travis/build/sabre-io/vobject/lib/Parser/XML.php:77
/home/travis/build/sabre-io/vobject/lib/Reader.php:91
/home/travis/build/sabre-io/vobject/tests/VObject/Parser/XmlTest.php:2793
/home/travis/build/sabre-io/vobject/tests/VObject/Parser/XmlTest.php:63

Needs a fix in sabre/xml first - which comes in sabre/xml 2.2

[phil-davis]

I restarted CI and now https://travis-ci.org/github/sabre-io/vobject/jobs/732473826
- Installing sabre/xml (2.2.2): Downloading (100%)

It finds the new sabre/xml 2.2.2 and the unit tests pass on PHP 8.0