Update diagnostic messages to ES2015 instead of ES6

[texastoland]

Also the messages for generator and async have slightly different wording:

1220: Generators are only available when targeting ECMAScript 6 or higher.
1311: Async functions are only available when targeting ECMAScript 6 and higher.

I only noticed trying to search the phrase EcmaScript 6 or higher.

[mhegazy]

sigh... ES6 is much easier to read than ES 2015.

PRs are welcomed. changes should be mainly in https://github.com/Microsoft/TypeScript/blob/master/src/compiler/diagnosticMessages.json and places in the code the report the errors.

[texastoland]

sigh... ES6 is much easier to read than ES 2015.

I agree but I noticed the configs were updated already. I'll PR momentarily.