Update the post_members function in the DoccanoClient to work with members endpoint

[david-engelmann]

Description

Currently, the post_members function fails because "rolename" isn't a field returned. The field has been updated to just "name". After that fix is implemented the next issue occurs because the project/{project_id}/roles endpoint is no longer valid. I've updated the url to reflect the new project/{project_id}/members endpoint and the data block to reflect what that endpoint needs.

Fixes # (https://github.com/doccano/doccano-client/issues/44)

[JessicaOjo]

@david-engelmann after the fix is made, no error is expected to occur, therefore the project/{project_id}/roles endpoint remains valid hence the data block should remain the same. Your current changes return an error.

[david-engelmann]

@JessicaOjo What version of Doccano are you using? The latest version doesn't have project/{project_id}/roles endpoint. Issues of these types are going to continue to happen as people using different versions of Doccano require different endpoints. A recent issue mentioned trying to figure out the doccano version and adjust based on that but I haven't been able to look into it yet.