Fix locale fa loading based on moment documentation.

[VahidBo]

Motivation

When you use the moment.loadPersian() function you will face an error that says the fa locale is not loaded.
Based on this document of moment this is a bug that prevents moment.locale from being loaded. And recommended solution is using this workaround:

var moment = require('moment');
require('moment/locale/cs');
console.log(moment.locale()); // cs

I applied this to solve the problem. And this PR also fix the #131.

And we want to change the fa locale using the moment.defineLocale function but we should use the moment.updateLocale() to change an existing locale. moment.defineLocale() should only be used for creating a new locale based on this.

[alitaheri]

This looks good 👍 But I'm not very familiar with the inner workings of moment.

@behrang This one needs your attention.

[behrang]

Sorry, I was very busy, and didn't find a time to look into this.