add main file mapping for jspm

[chenzhenxi]

I using JSPM to manage my dependence.

JSPM using SystemJS for loading library.

BUT, in index.js

var moment = module.exports = require("./moment-timezone");
moment.tz.load(require('./data/packed/latest.json'));

SystemJS didn't support require('./data/packed/latest.json')' for load json.

My fix:

I add JSPM options in package.json, make JSPM using builds/moment-timezone-with-data.js instead of index.js

Then everything is works for me.