x/text/currency: supported currency codes need an update (MRO->MRU, VEF->VES)

[elideveloper]

What version of Go are you using (go version)?

$ go version
go version go1.13.4 darwin/amd64

Does this issue reproduce with the latest release?

Yes.

What operating system and processor architecture are you using (go env)?

Any.

What did you do?

https://play.golang.org/p/GoUe1pIAVjo

What did you expect to see?

VES and MRU must be parsed.

What did you see instead?

currency: tag is not a recognized currency

Additional info

Since 2018 VEF->VES and MRO->MRU but currency lib supports old currency names.
https://www.iso.org/iso-4217-currency-codes.html
https://en.wikipedia.org/wiki/List_of_circulating_currencies

[ianlancetaylor]

CC @mpvl