Add MOODLE_404_STABLE to GHA/Travis tests #649

Job	Run time
CI test (make validate)	1m 1s
Code coverage (codecov)	1m 26s
Build moodle-plugin-ci.phar	29s
Integration tests (8.3, main)	6m 24s
Integration tests (8.3, MOODLE_404_STABLE)	6m 40s
Integration tests (8.2, MOODLE_403_STABLE)	6m 56s
Integration tests (8.2, MOODLE_402_STABLE)	5m 48s
Integration tests (8.1, MOODLE_401_STABLE)	7m 3s
Integration tests (8.0, MOODLE_400_STABLE)	6m 17s
Integration tests (8.0, MOODLE_311_STABLE)	5m 45s
Integration tests (7.4, MOODLE_39_STABLE)	5m 45s
SelfUpdate tests (PHAR) (8.3)	36s
Integration tests (PHAR) (8.3, main)	6m 3s
SelfUpdate tests (PHAR) (8.2)	30s
Integration tests (PHAR) (8.3, MOODLE_404_STABLE)	6m 11s
SelfUpdate tests (PHAR) (8.1)	21s
Integration tests (PHAR) (8.2, MOODLE_403_STABLE)	6m 5s
SelfUpdate tests (PHAR) (8.0)	21s
Integration tests (PHAR) (8.1, MOODLE_402_STABLE)	5m 36s
Integration tests (PHAR) (8.0, MOODLE_401_STABLE)	6m 24s
SelfUpdate tests (PHAR) (7.4)	15s
Integration tests (PHAR) (7.4, MOODLE_400_STABLE)	6m 12s
	1h 32m 8s

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