Updating a Server doesn't require :cores or :ram parameters

The `Server#update` method requires that the options `:cores` and `:ram` are provided while the [ProfitBricks API 1.2 documentation](https://www.profitbricks.com/de/en/iaas-developer-center/profitbricks-api/apidoc/APIDocumentation.html?UpdateServer.html) doesn't say this.

This pull request removes the mandatory check for the presence of `:cores` and `:ram` in `Server#update`.