Use nvm 0.39.7 #688

Job	Run time
CI test (make validate)	1m 20s
Code coverage (codecov)	1m 6s
Build moodle-plugin-ci.phar	27s
Integration tests (8.3, main, true)	6m 43s
Integration tests (8.3, MOODLE_404_STABLE, true)	6m 32s
Integration tests (8.2, MOODLE_403_STABLE, true)	6m 37s
Integration tests (8.2, MOODLE_402_STABLE, true)	6m 12s
Integration tests (8.1, MOODLE_401_STABLE, true)	6m 49s
Integration tests (8.0, MOODLE_400_STABLE)	5m 40s
Integration tests (8.0, MOODLE_311_STABLE)	5m 12s
Integration tests (7.4, MOODLE_39_STABLE)	4m 57s
SelfUpdate tests (PHAR) (8.3)	32s
SelfUpdate tests (PHAR) (8.2)	39s
Integration tests (PHAR) (8.3, main, true)	6m 14s
Integration tests (PHAR) (8.3, MOODLE_404_STABLE, true)	6m 16s
SelfUpdate tests (PHAR) (8.1)	25s
Integration tests (PHAR) (8.2, MOODLE_403_STABLE, true)	6m 23s
SelfUpdate tests (PHAR) (8.0)	31s
SelfUpdate tests (PHAR) (7.4)	19s
Integration tests (PHAR) (8.1, MOODLE_402_STABLE, true)	5m 27s
Integration tests (PHAR) (8.0, MOODLE_401_STABLE, true)	6m 47s
Integration tests (PHAR) (7.4, MOODLE_400_STABLE)	5m 0s
	1h 30m 8s

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