Remove orderedhash dep

[nikushi]

Hi,

In my understanding, we can remove the dependency for orderedhash since ruby(>= 1.9) supports ordered hash.

Note: this branch was forked from #17

[jeremywrnr]

I just installed this gem, and needed to install orderedhash in order to run it, since the latest published gem still requires it (lib/csv2json.rb:4). So I suggest either removing its usage and requirement, or adding it as a formal requirement to the gemspec.