Suppress a RuboCop offense

This PR suppresses the following RuboCop offense.

```console
% bundle exec rubocop
(snip)

Offenses:

spec/spec_helper.rb:12:4: C: [Correctable] Style/FetchEnvVar: Use
ENV.fetch('COVERAGE') or ENV.fetch('COVERAGE', nil) instead of ENV['COVERAGE'].
if ENV['COVERAGE'] == 'true'
   ^^^^^^^^^^^^^^^

251 files inspected, 1 offense detected, 1 offense auto-correctable
RuboCop failed!
```

spec/spec_helper.rb

@@ -9,7 +9,7 @@
 # in ./support/ and its subdirectories.
 Dir["#{File.dirname(__FILE__)}/support/**/*.rb"].sort.each { |f| require f }
 
-if ENV['COVERAGE'] == 'true'
+if ENV.fetch('COVERAGE', nil) == 'true'
   require 'simplecov'
   SimpleCov.start
 end