Ensure rubocop can run under Ruby 3.1 (capistrano#2099)

* Move de dependencies to Gemfile

* Pin psych gem so that rubocop runs on Ruby 3.1

Gemfile

@@ -3,6 +3,10 @@ source "https://rubygems.org"
 # Specify your gem's dependencies in capistrano.gemspec
 gemspec
 
+gem "mocha"
+gem "rspec"
+gem "rspec-core", "~> 3.4.4"
+
 group :cucumber do
   # Latest versions of cucumber don't support Ruby < 2.1
   # rubocop:disable Bundler/DuplicatedGem
@@ -12,8 +16,6 @@ group :cucumber do
     gem "cucumber"
   end
   # rubocop:enable Bundler/DuplicatedGem
-  gem "rspec"
-  gem "rspec-core", "~> 3.4.4"
 end
 
 # Latest versions of net-ssh don't support Ruby < 2.2.6
@@ -36,7 +38,9 @@ if Gem::Requirement.new("< 2.2").satisfied_by?(Gem::Version.new(RUBY_VERSION))
   gem "rake", "< 13.0.0"
 end
 
-# We only run danger once on a new-ish ruby; no need to install it otherwise
+# We only run danger and rubocop on a new-ish ruby; no need to install them otherwise
 if Gem::Requirement.new("> 2.4").satisfied_by?(Gem::Version.new(RUBY_VERSION))
   gem "danger"
+  gem "psych", "< 4" # Ensures rubocop works on Ruby 3.1
+  gem "rubocop", "0.48.1"
 end

capistrano.gemspec

@@ -31,8 +31,4 @@ Gem::Specification.new do |gem|
   gem.add_dependency "i18n"
   gem.add_dependency "rake", ">= 10.0.0"
   gem.add_dependency "sshkit", ">= 1.9.0"
-
-  gem.add_development_dependency "mocha"
-  gem.add_development_dependency "rspec"
-  gem.add_development_dependency "rubocop", "0.48.1"
 end