[FEATURE propertyBraceExpansion] Ensure __ember_observes__ is set properly.

The refactor in 5d95885 to allow only the declarative form of expansion
caused the `__ember_observes__` property to be unset when the feature
was enabled.

This was identified while fixing the Travis asset publishing and build
failures in #3863.

@twokul / @rjackson

/cc @hjdivad
Merge pull request #3875 from rjackson/fix_propertyBraceExpansion_prototype_expansion

[FEATURE propertyBraceExpansion] Ensure __ember_observes__ is set properly.