Relax dependency requirement on JSON

[Tereci]

Hi,
I am trying to migrate from JSON to HJSON for config files of my app and I am running into an issue with JSON gem compatibility as HJSON gem requires JSON ~> 2.2.0 while some of the other gems I am using are locked down on older versions of the JSON gem.

Would it be possible to relax the current requirement to a less strict that allows for older versions? I tried to build this gem locally with:
spec.add_runtime_dependency 'json', '>= 1.7.5'
and it seems to be working just fine. I can create a pull request if that would help.

[laktak]

@namusyaka could you take a look?

[namusyaka]

@Tereci Seems good to me. Please send a pull request.

[Tereci]

@namusyaka PL created, thank you.