Guard against loading moment-timezone twice

[mattjohnsonpint]

A few reports from the wild, such as in #193 and on StackOverflow (and others) have shown that it's easy to accidentally load moment-timezone twice, which creates some problems.

Usually, it's a misunderstanding that moment-timezone-with-data already contains moment-timezone.

We should either guard against this, or at least prevent it from causing problems.

[timrwood]

This is released in 0.4.0.