Clarify supported Node versions are >= 8.6 or 8.2.1 --harmony

@HoraceShmorace rightly comments that we have not made our conformance
to semver (or lack thereof) clear, and the Node support matrix change
in 1.2 constitutes an insufficiently-announced breaking change. This
commit updates the README and CHANGELOG to clarify that this library
will only run correctly on Node versions later than 8.2.1 `--harmony`,
not simply any version of Node v8.

Closes #419

Author: kadamwhite

CHANGELOG.md

@@ -20,7 +20,7 @@
 
 ## v1.2.0 _Space Is Only Noise_
 
-- **BREAKING**: The minimum supported node version is now v8.
+- **BREAKING**: The minimum supported node version is now v8.6, or v8.2.1 with the `--harmony` flag.
 - **BREAKING**: `._paging.total` and `._paging.totalPages` response properties are now returned as integers, not strings.
 - Bundled route handlers are now available for new first-party endpoints in WordPress 5.0.
 - The project now uses Jest and ESLint in place of Mocha, Chai, JSCS and JSHint. Thank you for your years of service, ["nyan" reporter](https://mochajs.org/#nyan)!

README.md

@@ -42,7 +42,7 @@ To get started, `npm install wpapi` or [download the browser build](https://wp-a
 
 ## Installation
 
-`node-wpapi` works both on the server or in the browser. Node.js version 8 or higher is required, and the latest LTS release is recommended.
+`node-wpapi` works both on the server or in the browser. Node.js version 8.6 or higher (or version 8.2.1 with the `--harmony` flag) is required, and the latest LTS release is recommended.
 
 In the browser `node-wpapi` officially supports the latest two versions of all evergreen browsers, and Internet Explorer 11.