fix: Missing rspec task in Rakefile when releasing

The release workflow runs without development and test gems installed,
but the rspec core is required to load the Rakefile.

Author: jgraichen

Gemfile

@@ -6,6 +6,7 @@ gemspec
 
 gem 'rake'
 gem 'rake-release', '~> 1.0'
+gem 'rspec', '~> 3.0'
 
 group :development do
   gem 'appraisal'
@@ -16,7 +17,6 @@ group :development do
 end
 
 group :test do
-  gem 'rspec', '~> 3.0'
   gem 'rspec-rails'
   gem 'simplecov'
   gem 'simplecov-cobertura'