Adhere to package naming conventions

[shadyvb]

The composer package name currently is elasticpress-stream instead of 10up/elasticpress-stream convention, which is quite confusing when you try to install it for the first time.

Updating the package name can break existing usage as this isn't published on packagist just yet, so you can't mark the existing name as abandoned, and people would be including it directly from GitHub, so further installations will error out. But It'd be great if this can be fixed in the next release.