Relax rails version in Gemfile

and, add sprockets-rails as a explicit development dependency.

Gemfile

@@ -10,10 +10,14 @@ gemspec
 # Git. Remember to move these dependencies to your gemspec before releasing
 # your gem to rubygems.org.
 
-gem 'rails', '>= 5.2.4.6', '< 6.2'
+gem 'rails', '>= 5.2.4.6', '< 7.1'
 
 group :development, :test do
   gem 'pry'
   gem 'pry-nav'
   gem 'rubocop'
+
+  # Since rails 7.0, rails does not require sprockets-rails.
+  # This is added to run the same tests as in previous versions.
+  gem 'sprockets-rails'
 end