Fix building tasks for moment-with-locales.js and moment-with-locales.custom.js

[dennisgl]

#2367 moment-with-locales.custom.js uses last defined locale by default

moment-with-locales.js and moment-with-locales.custom.js were default to "en".

See grunt task embedLocales:

var languageReset = 'moment.locale(\'en\');';

Adding it back to transpiling task seems the right thing to do.

---

custom locale building was broken

In current built moment-with-locales.custom.js:

this line

var moment_with_locales_custom = moment;

which moment is actually undefined, and should be replaced with the matches from the regex,
and thus become

var moment_with_locales_custom = moment__default;

[dennisgl]

@ichernev, I would be glad if you could take a look on the PR, and let me know if the changes are appropriate 🌻

[wyantb]

👍 LGTM. Fixes #2399 #2367 etc. @ichernev looking at this? Would be great for a 2.10.4.

[mattjohnsonpint]

@timrwood - this one is important. Can you please take a look and push out a build if possible. Thanks.

[mattjohnsonpint]

I'm merging this now so it gets in the next build. Thanks.