install ember-tether in docs app #576

Job	Run time
Floating Dependencies	1m 12s
Tests - Basic App	4m 5s
Tests	1m 10s
Tests - App That Excludes Mirage	1m 0s
Docs	1m 23s
ember-lts-3.28	1m 10s
ember-lts-4.4	1m 1s
ember-lts-4.8	1m 5s
ember-lts-4.12	1m 19s
ember-release	1m 1s
ember-beta	1m 5s
ember-canary	1m 54s
	17m 25s

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