use more standard postcss process #403

Job	Run time
Floating Dependencies	1m 43s
Percy Tests	2m 14s
Tests	2m 46s
ember-canary - Allowed to fail	3m 7s
ember-lts-3.28	2m 50s
ember-release-no-deprecations - Allowed to fail	2m 51s
ember-release	2m 59s
ember-beta	3m 4s
ember-classic	2m 57s
ember-default-with-jquery	2m 55s
embroider-safe	3m 11s
embroider-optimized	3m 10s
no-deprecations	2m 59s
	36m 46s

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