Release name is not required by underlying API

Despite the [underlying API](https://developer.github.com/v3/repos/releases/#create-a-release) not requiring a release name to be specified, [this module still requires it](https://github.com/actions/create-release/blob/master/src/create-release.js#L17). It seems to me this module should be a thin wrapper around the underlying API, which means it should not change the rules about what is required.