Remove the dependency on the _a and _tzm properties from moment core.

[timrwood]

[naturalethic]

This might be propagated to bug.

I found that in some situations (a during protractor test in my case), moment construction does not populate the _a property, causing needsOffset to return false. I found that if I make sure to pass a format to moment.tz (thus to moment.utc) it would populate _a in my specific case. Because that resolved my issue I didn't investigate further. I report it here for your consideration.