support for ruby 2.0

[tbarbugli]

Hi,
the gem is using required keywords arguments which is a feature introduced in 2.1. Would you consider merging a PR that switches to "normal" keywords arguments?

[pda]

I'm much more open to supporting 2.0 than I was to 1.9.x. What would that look like?

1. foo(arg:) becomes foo(arg: nil)
   • 👍 simple.
   • 👎 we lose the assurance that the argument was passed.
2. foo(arg:) becomes foo(arg: nil) with argument assertion in the method body.
   • 👍 can replicate the current behavior.
   • 👎 method bodies become verbose, boilerplate.
   • 👎 can't distinguish between argument passed as nil and argument not passed.
   • not bad overall.
3. foo(arg:) becomes foo(arg: raise(ArgumentError, :arg))
   • 👍 can replicate the current behavior.
   • 👍 method bodies don't need to change.
   • 👎 method declarations become very long with multiple arguments.
4. Something else…?