Fix: Properly require ostruct

[strzibny]

This properly requires OpenStruct.

$ irb -Ilib
irb(main):001:0> require 'jbuilder'
NameError: uninitialized constant OpenStruct
    from /home/strzibny/projects/jbuilder/lib/jbuilder.rb:26:in `<class:Jbuilder>'
    from /home/strzibny/projects/jbuilder/lib/jbuilder.rb:7:in `<top (required)>'
    from /usr/share/rubygems/rubygems/core_ext/kernel_require.rb:55:in `require'
    from /usr/share/rubygems/rubygems/core_ext/kernel_require.rb:55:in `require'
    from (irb):1
    from /usr/bin/irb:11:in `<main>'

[rwz]

Thanks!