ci: track coverage with codecov.io instead of codeclimate.com.

Comparison:

* Codecov has a simpler integration and a better UI.
* Codeclimate tracks both linter warnings (Rubocop, ESLint) and code
  coverage, but its UI for code coverage is worse than Codecov's.
* Codeclimate doesn't support Simplecov 0.18 because Codeclimate doesn't
  support 0.18's new coverage format yet.

.simplecov

@@ -5,4 +5,10 @@ SimpleCov.start "rails" do
   #minimum_coverage line: 85, branch: 75
   #minimum_coverage_by_file 50
   #coverage_dir "tmp/coverage"
+
+  # https://github.com/codecov/codecov-ruby#submit-only-in-ci-example
+  if ENV["CODECOV_TOKEN"]
+    require "codecov"
+    SimpleCov.formatter = SimpleCov::Formatter::Codecov
+  end
 end