Feature: Support multiple project slugs

[brentobi]

Hey there,

First of all: Thanks for this repo!

This PR:

• allows that by supporting to set a whitespace separated list of project slugs (set :sentry_project, 'my-foo my-bar'),
• fixes existing rubocop offenses because I assumed that the existing .rubocop.yml is not deprecated and
• sets a required ruby version via spec.required_ruby_version = '>= 2.5.0' because I noticed that the code would fail running on 2.4.9 because of String.delete_suffix usage.

Reasoning: I found myself in a position where I have separate frontend and backend projects in Sentry (to allow users to fine tune their notifications more easily), but one git repository for both project's code ("historical reasons"). Therefore I need to notify Sentry about one release being deployed to both projects. This PR intends to support exactly that.

Please let me know if i should split up these changes into several PRs or if any other changes are required.

Thanks!

[brentobi]

Opening a PR on master was not that smart of me.