(maint) pin rubocop dependencies to work on ruby 2.1

Gemfile

@@ -12,13 +12,20 @@ group :tests do
   gem 'license_finder' if Gem::Version.new(RUBY_VERSION) >= Gem::Version.new('2.4.0')
   gem 'rake', '~> 10.0'
   gem 'rspec', '~> 3.0'
-  gem 'rubocop-rspec'
   # rubocop 0.58 throws when testing against ruby 2.1, so pin to the latest,
   # unless we're dealing with jruby...
   if RUBY_PLATFORM == 'java'
-    gem 'rubocop'
-  else
+    # don't install rubocop on JRuby - we don't need it
+  elsif Gem::Version.new(RUBY_VERSION) < Gem::Version.new('2.2.0')
     gem 'rubocop', '0.57.2'
+    # the last version of parallel to support ruby 2.1
+    gem 'parallel', '1.13.0'
+    gem 'rubocop-rspec'
+  else
+    # 2.1-compatible analysis was dropped after version 0.58
+    # This needs to be removed once we drop puppet4 support.
+    gem 'rubocop', '~> 0.57.0'
+    gem 'rubocop-rspec'
   end
   gem 'simplecov-console'
   # the test gems required for module testing