Upgrade Ruby to 2.3.1
Upgrade Ruby to 2.3.1

- Updated Ruby to 2.3.1 as Ruby 2.1 was only getting security updates.

Note that this commit results in a warning being generated in the tests:
  `[warning] The response contained in an RestClient::Exception is now a RestClient::Response instead of a Net::HTTPResponse, please update your code`

This is caused by gds-api-adapters when it converts an error response into
 json for log purposes. It still actually does what we expect and appears
 to be on the rest-client radar as [rest-client/rest-client#454](https://github.com/rest-client/rest-client/issues/454)
 due to be fixed in rest-client 2.x

alphagov/business-support-api,alphagov/business-support-api,alphagov/business-support-api,alphagov/business-support-api