Fix missing RSpec in SMTP pool CI

Installs development dependencies from the Gemfile

vendor/gems/mail-smtp_pool/.gitlab-ci.yml

@@ -12,6 +12,7 @@ workflow:
     - ruby -v                                   # Print out ruby version for debugging
     - gem install bundler --no-document         # Bundler is not installed with the image
     - bundle config set --local path 'vendor'   # Install dependencies into ./vendor/ruby
+    - bundle config set with 'development'
     - bundle install -j $(nproc)
   script:
     - bundle exec rspec