change from GET to POST /logout with _method=delete

change to using POST /logout with _method=delete
Not that I can see:
https://www.adobe.com/devnet/photoshoplightroom/sdk/eula_lr5.html

(Yes, you need to agree to an EULA, download an archive, then open the HTML file inside to read the API docs. Web 2.0 just can't handle the awesome inside.)