Require Ruby 2.2.6+

[tarcieri]

• Prior major Ruby versions are EOL or soon-to-be EOL
• 2.2.6 is the latest release of the 2.2 series

[elijh]

For what it is worth, the stable version of Debian still has ruby 2.1.5p273

[tarcieri]

That would put it 5 releases behind the last 2.1 release (2.1.10), which was in April, and is likely to be the last release of the 2.1 series before it is EOL'd next February. I'd also note it's incapable of running Rails 5.

Like ruby-core said February 2016:

We highly recommend that you upgrade to Ruby 2.3 or 2.2 as soon as possible.

That said, if there are changes you'd like backported to RbNaCl 3.x (this PR will likely be incorporated into a 4.0.0 release) that's doable.