Add support for LinkedIn v2 API.

[mdesjardins]

The current implementation breaks when it fetches the additional raw_info because it hits a v1 endpoint to do so. Unfortunately LinkedIn disables the v1 API when they upgrade clients to v2, so authentication with this gem will stop working if/when LinkedIn does this.

[mdesjardins]

PR submitted #38

[kalemi19]

With LinkedIn officially deprecating V1 on March 2019, this needs to be taken care of, otherwise this gem will become obsolete.

[pjg]

Superceded by #44. As such, I believe this issue could be closed.