ES6 data task updates

[timrwood]

This changes the data compilation process in a few ways.

grunt data-tests

Only exports tests for the current year for each timezone. Before #82, a full battery of tests for each timezone was useful, as we were jumping through all the rules and zones and doing complicated logic. Now that we use a precomputed list and generate the tests from a precomputed list, the tests are sort of redundant.

These tests are grouped together into one file now. https://github.com/timrwood/moment-timezone/blob/es6-build-data/test/zone/changes.js This will make it easier to visually diff changes for the year for a tzdb update.

grunt data-dedupe

Move the unpacked json files into the temp folder so that they are no longer tracked in git. They weren't used for anything and made diffs noisy.

grunt data-pack

Store packed data in the src folder instead of the data folder.

src/data/archives/2016a.json
src/data/archives/2016b.json
src/data/archives/2016c.json
src/data/latest.json
src/data/latest.js
src/data/latest-2010-2020.js

grunt data-meta

Store meta data in the src folder as src/data/meta.json.

For legacy purposes, I copied the old tasks that these replaced to a similarly named task with -legacy appended. Once the conversion is complete, we can remove these tasks.