Upgrade json-to-ast to a stable release

[postatum]

Currently it's version 2.0.0-alpha1.3 because it's the last version which doesn't add breaking changes.

First release to add breaking changes is 2.0.0: https://github.com/vtrushin/json-to-ast/releases/tag/v2.0.0

---

To see the difference in generates json-ast:

1. Generate json-ast with version 2.0.0-alpha1.3 and options {verbose: true}.
2. Generate json-ast with version 2.1.0 or 2.0.0 and options {loc: true}.
3. Compare.

---

Currently upgrading to 2.1.0 makes 95 tests in raml-js-parser-2 fail when running them travis-like:

npm run devInstall
npm run buildall && npm run testall