consistent quoting, more readable interpolation

via http://refactorcop.com/rspec/rspec-rails

Author: josephgrossberg

features/support/rubinius.rb

@@ -1,5 +1,5 @@
 # Required until https://github.com/rubinius/rubinius/issues/2430 is resolved
-ENV['RBXOPT'] = "#{ENV["RBXOPT"]} -Xcompiler.no_rbc"
+ENV['RBXOPT'] = "#{ENV['RBXOPT']} -Xcompiler.no_rbc"
 
 Around "@unsupported-on-rbx" do |scenario, block|
   block.call unless defined?(Rubinius)