Properly set Cache-Control on CDN hosting

[jeffposnick]

As per googleapis/google-cloud-node#1087 (comment), perhaps this will work?

[workbox-pr-bot]

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File	Before	After	Change	GZipped
packages/workbox-background-sync/build/workbox-background-sync.prod.js	3.51 KB	3.51 KB	0%	1.45 KB
packages/workbox-broadcast-cache-update/build/workbox-broadcast-cache-update.prod.js	1.12 KB	1.12 KB	0%	584 B
packages/workbox-build/build/_types.js	41 B	41 B	0%	61 B
packages/workbox-build/build/index.js	4.02 KB	4.02 KB	0%	1.57 KB
packages/workbox-cache-expiration/build/workbox-cache-expiration.prod.js	3.88 KB	3.88 KB	0%	1.37 KB
packages/workbox-cacheable-response/build/workbox-cacheable-response.prod.js	587 B	587 B	0%	345 B
packages/workbox-cli/build/app.js	6.50 KB	6.50 KB	0%	2.16 KB
packages/workbox-cli/build/bin.js	2.32 KB	2.32 KB	0%	1.03 KB
packages/workbox-core/build/workbox-core.prod.js	7.12 KB	7.12 KB	0%	2.81 KB
packages/workbox-google-analytics/build/workbox-google-analytics.prod.js	2.12 KB	2.12 KB	0%	1.03 KB
packages/workbox-precaching/build/workbox-precaching.prod.js	5.57 KB	5.57 KB	0%	2.15 KB
packages/workbox-range-requests/build/workbox-range-requests.prod.js	1.63 KB	1.63 KB	0%	797 B
packages/workbox-routing/build/workbox-routing.prod.js	2.87 KB	2.87 KB	0%	1.30 KB
packages/workbox-strategies/build/workbox-strategies.prod.js	4.30 KB	4.30 KB	0%	1.14 KB
packages/workbox-streams/build/workbox-streams.prod.js	1.66 KB	1.66 KB	0%	764 B
packages/workbox-sw/build/workbox-sw.js	1.47 KB	1.47 KB	0%	791 B
packages/workbox-webpack-plugin/build/generate-sw.js	8.04 KB	8.04 KB	0%	2.57 KB
packages/workbox-webpack-plugin/build/index.js	742 B	742 B	0%	470 B
packages/workbox-webpack-plugin/build/inject-manifest.js	10.30 KB	10.30 KB	0%	3.23 KB

Workbox Aggregate Size Plugin

9.68KB gzip'ed (65% of limit)
25.18KB uncompressed

[coveralls]

Coverage remained the same at 86.833% when pulling 02b49b7 on cache-control-cdn into a432b25 on master.