Drop Moment and Moment Timezone

[blowery]

We're not supplying any custom translations for these and having them as dependencies forces them on clients of this module. Both are quite large when bundled with webpack and would be better included by a client project if they wish to use them.

[beaucollins]

At the very least relax the version requirement. We're using i18n-calypso in a project that also uses moment-timezone and now we have two moment-timezones:

moment-timezone@^0.5 seems reasonable.

[beaucollins]

I see #65 already deletes the dependency now!