Update to moment-timezone 0.0.6

We're currently on the 0.0.3. Between 0.0.3 and 0.0.6, there were a number of bugfixes and performance improvements: https://github.com/moment/moment-timezone/blob/develop/changelog.md

**Important note:** In order to do this upgrade, we'll also have to bump Moment up from 2.5.0 to 2.7.0. I don't think that should pose any problems: https://github.com/moment/moment/blob/develop/CHANGELOG.md Unless any of our Node apps are relying on that `moment` global? Might be worth double-checking...

I'd like to get up to moment-timezone 0.1.0, but that contains breaking changes, which I think would warrant a new major version of I18n. By contrast, upgrading to moment-timezone 0.0.6—the last stable version before 0.1.0—should be a painless bump.