Drop support for ruby 2.0 (EOL'd March 2016), also possibly ruby 2.1?

[spovich]

@kshnurov any concern about dropping ruby 2.0 support? It was EOL'd in early 2016.

We may also consider dropping ruby 2.1 since rails 5.0+ requires ruby 2.2+.

Thoughts?

[kshnurov]

@spovich sure, we can drop them if they cause any problems, but I don't see any reasons for making it right now. Is there some?

[spovich]

I just noticed during gem updates that the latest rubocop only supports ruby 2.1+. Rubocop 0.50.0 is the last version supporting 2.0. It's definitely not a big deal :) I would like to add 2.4 and 2.5 to the travis matrix. I'll do that now.