put-instead-of-post-client

- `Gemfile.lock` is updated to rely on the newest version of all Gem
  dependencies.

- `Client.close_usage_event`, `Client.open_usage_event`,
  `Client.open_app_ownership_event` and
  `Client.close_app_ownership_event` make PUT requests and expect
  *HTTP 201 Created* responses for successful requests.

See heroku/vault-usage#35 for the equivalent change in the server.