Move phpunit-polyfills to dev-requirements #146

Job	Run time
Coding Standards (7.4, locked)	24s
Tests on unsupported PHP (7.1, lowest)	11s
Static Code Analysis (7.4, locked)	27s
Tests on unsupported PHP (7.1, highest)	12s
Tests (8.0, lowest)	56s
Tests on unsupported PHP (7.0, lowest)	12s
Tests (8.0, locked)	20s
Tests on unsupported PHP (7.0, highest)	13s
Tests (8.0, highest)	20s
Tests on unsupported PHP (5.6, lowest)	10s
Tests (7.4, lowest)	10s
Tests on unsupported PHP (5.6, highest)	12s
Tests (7.4, locked)	10s
Tests (7.4, highest)	16s
Tests (7.3, lowest)	15s
Tests (7.3, locked)	11s
Tests (7.3, highest)	16s
Tests (7.2, lowest)	17s
Tests (7.2, highest)	11s
	5m 23s

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