Prevent Shakapacker v7 deprecation messages #1834

Job	Run time
build-dummy-app-webpack-test-bundles (2.7, 14)	4m 12s
build-dummy-app-webpack-test-bundles (2.7, 20)	3m 55s
build-dummy-app-webpack-test-bundles (3.2, 14)	1m 42s
build-dummy-app-webpack-test-bundles (3.2, 20)	1m 23s
main (2.7, 14, run_rspec:all_but_examples)	11m 9s
main (2.7, 14, run_rspec:examples)	9m 54s
main (2.7, 20, run_rspec:all_but_examples)	10m 55s
main (2.7, 20, run_rspec:examples)	9m 10s
main (3.2, 14, run_rspec:all_but_examples)	7m 44s
main (3.2, 14, run_rspec:examples)	5m 46s
main (3.2, 20, run_rspec:all_but_examples)	6m 31s
main (3.2, 20, run_rspec:examples)	5m 6s
	1h 17m 27s

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