[bugfix] Handle invalid mutations

[Alanscut]

fixes issue #5075

Summary

Add and substract can change the original moment, moment._ Isvalid should change synchronously.

Test plan

src/test/moment/is_valid.js

[coveralls]

Coverage increased (+0.004%) to 88.489% when pulling 9ab2dc2 on Alanscut:issue_5075 into a0f3c74 on moment:develop.

[marwahaha]

This will recompute isValid every time it runs.
Perhaps could we compute _isValid whenever the moment object is mutated?
@ichernev may also have input.

[marwahaha]

I'm not sure we have to assign these; could we do

var flags,
  parsedParts,
  isNowValid = ...

[ichernev]

Merged in 5718d97