Drop support of ruby 1.9.x and 2.0.x

[sachin21]

Drop support 1.9.x and 2.0.x. because nokogiri requires Ruby version >= 2.1.0.

[sachin21]

@eagletmt How about this?

[r7kamura]

Hi @sachin21, thank you for your contribution 👍

It looks good change for me, because supporting old and different Ruby versions can be a hard task for us. In this opportunity, we might want to re-think about supported ruby versions and gemspec's required_ruby_version= directive.

@eudoxa @eagletmt @taiki45 What do you think of this? In fact, I don't have any projects that use this library anymore, so the authority to decide this change should be owned by you, the cookpad developers.

[eagletmt]

+1 for dropping support for Ruby <= 2.0.x.
@sachin21 Could you add gem.required_ruby_version = '>= 2.1.0' ?

[taiki45]

Make sense to me and +1.

[sachin21]

Thank you all. and thank you for your review.
I added gem.required_ruby_version = '>= 2.1.0' and I think this is huge changes so I updated CHANGELOG.md.

[taiki45]

You can say "Drop support of ruby 1.9.x and 2.0.0.". Don't miss period at the end of sentence.

And I also found we mention ruby version in README, so let's change it.

[sachin21]

My bad. Thanks. I fixed it.

[sachin21]

Thank you so much.