Upgrade to PHPUnit 8; bump required PHP to 7.2+; fix Travis tests on PHP 7.4

[osma]

We have been using obsolete PHPUnit 7, which is not compatible with PHP 7.4 and is no longer supported.
This PR upgrades to PHPUnit 8, which is still supported (but only in Life Support mode).
It requires PHP 7.2, so this PR drops support for PHP 7.1 and attempts to get Travis CI tests working on PHP 7.4.

Some tests had to be modified (add void return types for setUp and tearDown methods) to keep PHPUnit 8 happy. There are warnings about deprecated PHPUnit features that should be addressed as well but they are not fatal (yet).

Fixes #920

[codecov]

Codecov Report

Merging #1127 (adbe7ac) into master (1fc53b3) will increase coverage by 7.52%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master    #1127      +/-   ##
============================================
+ Coverage     60.38%   67.90%   +7.52%     
+ Complexity     1584     1583       -1     
============================================
  Files            32       32              
  Lines          4377     3889     -488     
============================================
- Hits           2643     2641       -2     
+ Misses         1734     1248     -486     

Impacted Files	Coverage Δ	Complexity Δ
model/DataObject.php	77.50% <100.00%> (-0.55%)	22.00 <0.00> (-1.00)
model/Model.php	83.68% <100.00%> (+2.15%)	105.00 <0.00> (ø)
model/VocabularyCategory.php	93.33% <100.00%> (ø)	7.00 <0.00> (ø)
model/sparql/GenericSparql.php	92.12% <100.00%> (+24.19%)	319.00 <0.00> (ø)
model/ConceptPropertyValue.php	86.74% <0.00%> (-1.21%)	47.00% <0.00%> (ø%)
model/Request.php	67.07% <0.00%> (+0.80%)	50.00% <0.00%> (ø%)
model/VocabularyConfig.php	94.00% <0.00%> (+0.82%)	89.00% <0.00%> (ø%)
model/PluginRegister.php	70.14% <0.00%> (+1.03%)	28.00% <0.00%> (ø%)
model/Vocabulary.php	87.85% <0.00%> (+1.05%)	112.00% <0.00%> (ø%)
... and 16 more

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[osma]

Tests are still failing on PHP 7.4. I think EasyRdf needs to be upgraded first...

[osma]

EasyRdf update in PR #1128 needs to be merged first, then I can continue working on this PR.

[osma]

Rebased on current master branch and force-pushed.

The PHPUnit 8 deprecation warnings should now be fixed; waiting for Travis CI tests to finish.

[sonarcloud]

SonarCloud Quality Gate failed.

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
9.3% Duplication

[kouralex]

Excellent PR! This indeed fixes Travis tests for PHP7.4. Testing with PHP 7.2, everything is working.

I did not notice any regressions in code changes, only improvements.

I will mark this PR ready for merging. Good job!