Cap rake and json_pure versions on Ruby < 2.0

Gemfile

@@ -7,7 +7,18 @@ else
 end
 
 gem 'puppetlabs_spec_helper', :require => false
-# https://github.com/rspec/rspec-core/issues/1864
-gem 'rspec', '< 3.2.0', {"platforms"=>["ruby_18"]}
-gem 'rspec-puppet', '~> 2.1.0', :require => false
-gem 'metadata-json-lint',       :require => false
+gem 'rspec-puppet',           :require => false
+gem 'metadata-json-lint',     :require => false
+
+## compatibility
+
+platforms :ruby_18 do
+  # https://github.com/rspec/rspec-core/issues/1864
+  gem 'rspec', '< 3.2.0',  :require => false
+  gem 'rake',  '< 11.0.0', :require => false
+end
+
+platforms :ruby_18, :ruby_19 do
+  gem 'json',      '< 2.0.0', :require => false
+  gem 'json_pure', '< 2.0.0', :require => false
+end