Fixes for Ruby 2.2.0

For rubby 2.2.0.
The problem is, without that conditional we end up with a not defined error when booting:
```
/home/vagrant/src/shopify/.bundle/ruby/2.2.0/bundler/gems/syck-e118f29d2227/lib/yaml/engine_manager.rb:48:in `remove_const': constant Psych::ENGINE not defined (NameError)
        from /home/vagrant/src/shopify/.bundle/ruby/2.2.0/bundler/gems/syck-e118f29d2227/lib/yaml/engine_manager.rb:48:in `<module:YAML>'
        from /home/vagrant/src/shopify/.bundle/ruby/2.2.0/bundler/gems/syck-e118f29d2227/lib/yaml/engine_manager.rb:16:in `<top (required)>'
        from /home/vagrant/src/shopify/.bundle/ruby/2.2.0/bundler/gems/syck-e118f29d2227/lib/syck.rb:10:in `require'
        from /home/vagrant/src/shopify/.bundle/ruby/2.2.0/bundler/gems/syck-e118f29d2227/lib/syck.rb:10:in `<top (required)>'
        from /usr/lib/shopify-ruby/2.2.0/lib/ruby/gems/2.2.0/gems/bundler-1.7.3/lib/bundler/runtime.rb:76:in `require'
        from /usr/lib/shopify-ruby/2.2.0/lib/ruby/gems/2.2.0/gems/bundler-1.7.3/lib/bundler/runtime.rb:76:in `block (2 levels) in require'
        from /usr/lib/shopify-ruby/2.2.0/lib/ruby/gems/2.2.0/gems/bundler-1.7.3/lib/bundler/runtime.rb:72:in `each'
        from /usr/lib/shopify-ruby/2.2.0/lib/ruby/gems/2.2.0/gems/bundler-1.7.3/lib/bundler/runtime.rb:72:in `block in require'
        from /usr/lib/shopify-ruby/2.2.0/lib/ruby/gems/2.2.0/gems/bundler-1.7.3/lib/bundler/runtime.rb:61:in `each'
        from /usr/lib/shopify-ruby/2.2.0/lib/ruby/gems/2.2.0/gems/bundler-1.7.3/lib/bundler/runtime.rb:61:in `require'
        from /usr/lib/shopify-ruby/2.2.0/lib/ruby/gems/2.2.0/gems/bundler-1.7.3/lib/bundler.rb:133:in `require'
        from /home/vagrant/src/shopify/config/application.rb:11:in `<top (required)>'
        from /home/vagrant/src/shopify/.bundle/ruby/2.2.0/gems/railties-4.0.12/lib/rails/commands.rb:60:in `require'
        from /home/vagrant/src/shopify/.bundle/ruby/2.2.0/gems/railties-4.0.12/lib/rails/commands.rb:60:in `<top (required)>'
        from script/rails:24:in `require'
        from script/rails:24:in `<main>'
```