Upgrade phpunit

[mvorisek]

Tested with:

• latest phpunit 5
• latest phpunit 6
• latest phpunit 7
• latest phpunit 8
• latest phpunit 9 (PHP 7.2 is not supported, ok)

[codecov]

Codecov Report

Merging #47 into develop will decrease coverage by 12.15%.
The diff coverage is 100.00%.

@@              Coverage Diff               @@
##             develop      #47       +/-   ##
==============================================
- Coverage      74.03%   61.87%   -12.16%     
  Complexity       157      157               
==============================================
  Files              6        6               
  Lines            362      362               
==============================================
- Hits             268      224       -44     
- Misses            94      138       +44     

Impacted Files	Coverage Δ	Complexity Δ
src/PhpunitTestCase.php	84.28% <100.00%> (ø)	29.00 <3.00> (?)
src/Migration.php	62.55% <0.00%> (-18.73%)	105.00% <0.00%> (ø%)
src/MigratorConsole.php	0.00% <0.00%> (ø)	4.00% <0.00%> (ø%)

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