Use keyword arguments instead of options hash

[styd]

Prefer to use keyword arguments to options hash for these reasons:

• Fewer characters or lines of code to type.
• We can immediately discover the names of the arguments without having to read the body of the method.
• Less chance of typos if in the future we want to add new arguments since we don’t have to write the boilerplate code to extract hash options.

Source: Ruby 2 Keyword Arguments

[radar]

This would break compatibility with versions older than 2.0.

I am completely fine with that.

People have had enough time to upgrade.

You should force the Ruby version to be greater than 2.0 in the gemspec to avoid any surprises.

[styd]

Yes, exactly. I'm sure most people already use Ruby 2 because versions older than that are no longer supported long ago. In fact, even support for Ruby 2.1 has ended on April this year. Ok, I'm gonna merge this. Thanks, @radar.
Anyway, I think these changes deserved a version upgrade, don't you think?