Use platforms instead of conditionals in Gemfile

Gemfile

@@ -27,10 +27,13 @@ gem 'dalli', '>= 2.2.1'
 local_gemfile = File.dirname(__FILE__) + "/.Gemfile"
 instance_eval File.read local_gemfile if File.exists? local_gemfile
 
-platforms :mri do
-  group :test do
-    gem 'ruby-prof', '~> 0.11.2' if RUBY_VERSION < '2.0'
-    gem 'debugger' if !ENV['TRAVIS'] && RUBY_VERSION < '2.1'
+group :test do
+  platforms :mri_19 do
+    gem 'ruby-prof', '~> 0.11.2'
+  end
+
+  platforms :mri_19, :mri_20 do
+    gem 'debugger' if !ENV['TRAVIS']
   end
 end