Add the libonig5 to the Travis packages to install #515

Job	Run time
CI test (make validate)	39s
Code coverage (codecov)	32s
Build moodle-plugin-ci.phar	37s
Integration tests (8.2, master)	6m 32s
Integration tests (8.2, MOODLE_403_STABLE)	6m 24s
Integration tests (8.2, MOODLE_402_STABLE)	7m 49s
Integration tests (8.1, MOODLE_401_STABLE)	5m 58s
Integration tests (8.0, MOODLE_400_STABLE)	7m 8s
Integration tests (8.0, MOODLE_311_STABLE)	8m 5s
Integration tests (7.4, MOODLE_310_STABLE)	7m 27s
Integration tests (7.4, MOODLE_39_STABLE)	5m 9s
Integration tests (7.4, MOODLE_38_STABLE)	6m 16s
Integration tests (PHAR) (8.2, master)	5m 23s
Integration tests (PHAR) (8.1, MOODLE_403_STABLE)	6m 21s
Integration tests (PHAR) (8.0, MOODLE_401_STABLE)	5m 56s
Integration tests (PHAR) (7.4, MOODLE_400_STABLE)	5m 22s
	1h 25m 38s

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