ember-flatpickr now observes changes to altFormat attr

[Techn1x]

Fixes #255

So the test actually fails because flatpickr itself doesn't currently update altFormat for some reason. I have made them aware of the issue here;
flatpickr/flatpickr#1431

So this PR is just in preparation for when that issue is fixed - it should work once that's done.

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Unrelated, a different test about locales was failing too

not ok 16 Chrome 67.0 - Integration | Component | ember flatpickr: locale works correctly
actual: > décembre
expected: >Décembre