Integrate coveralls with github actions

Somehow I thought it worked automatically, but we had to provide the
token.

The configuration is based on Coveralls instructions to run parallel
builds [1].

Alternatively we could use the Coveralls GitHub Action [2] which
slightly simplifies the workflow configuration and removes the
dependency of the coveralls gem. However, it also adds a dependency on
simplecov-lcov and requires configuring it to renerate LCOV files on
each run, so the benefits of using it are not that big.

[1] https://docs.coveralls.io/parallel-build-webhook
[2] https://github.com/coverallsapp/github-action/

spec/rails_helper.rb

@@ -1,5 +1,5 @@
 ENV["RAILS_ENV"] ||= "test"
-if ENV["COVERALLS"]
+if ENV["COVERALLS_REPO_TOKEN"]
   require "coveralls"
   Coveralls.wear!("rails")
 end