[Nexmo/oas_parser#20] alternative implementation for resolving remote…

[kfalconer]

This is an alternative implementation for issue #20 which will work for both a whole document remote reference

$ref: 'document.json'

and for local references contained within a remote reference

$ref: 'document.json#/myElement'

Additional notes about changes included in this PR:

• There is no support for URL references, an error will be raised if found
• There are likely more edge cases supported by the parser, but only a moderately complex version was tested.
• In addition to the unit test, I've also test with ReDoc
• The version dependencies for nokogiri and activesupport have been relaxed because it is best practice to require the minimum version for functionality to work.

[mheap]

Thanks for this @kfalconer, that's released as oas_parser-0.16.0.gem