Move to MultiJson for jruby compatibility

[colonel-sanders]

Despite the lead author's performance concerns, Oj isn't available for all interpreters. Proposing a switch to MultiJson to keep this gem broadly compatible.

[colonel-sanders]

Upon reflection, considering a different approach

[xentek]

Other than the use of mode: :compat with MultiJson (since I'm not 100% sure MultiJson.dump can take options, nor would do anything with the Oj option passed as is), I would welcome this pull request.

We are currently using MRI but considering a move to rbx and/or jruby, so would need to update our gems to use MultiJson anyway. Thanks for the patch in either case, even if you recalled it :)

[colonel-sanders]

MultiJson should pass along options hashes (at least in more recent versions) to the underlying implementation. Aside from that, I'm no longer convinced that MultiJson is the best implementation choice here given its questionable future. Perhaps gson.rb under jruby and Oj under all C-extension-compatible platforms?

However, that's not the main problem with my PR.

More significantly, the code doesn't work under jruby. The jruby ext for do-postgres attempts to bind the json column values as regular varchars. Postgres steadfastly refuses to make the implicit conversion when it's explicitly bound this way.

I'll come back around with a new PR as soon as I get the code actually working properly.