Prevent Shakapacker v7 deprecation messages #1864

Job	Run time
build-dummy-app-webpack-test-bundles (3, 16)	1m 24s
build-dummy-app-webpack-test-bundles (3, 20)	1m 14s
build-dummy-app-webpack-test-bundles (3.3, 16)	1m 16s
build-dummy-app-webpack-test-bundles (3.3, 20)	1m 16s
main (3, 16, run_rspec:all_but_examples)	7m 33s
main (3, 16, run_rspec:examples)	4m 38s
main (3, 20, run_rspec:all_but_examples)	6m 56s
main (3, 20, run_rspec:examples)	5m 16s
main (3.3, 16, run_rspec:all_but_examples)	7m 30s
main (3.3, 16, run_rspec:examples)	5m 3s
main (3.3, 20, run_rspec:all_but_examples)	7m 8s
main (3.3, 20, run_rspec:examples)	4m 49s
	54m 3s

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