Add SimpleCov into the mix to get C0 coverage stats.

Plug in SimpleCov, as even the minimal guarantees provided by C0 coverage can prove handy for a security-related tool.
Note the `|| (version[0] >= 2)` portion.

It's restricted to Ruby 1.9.x, or 2.0.0+ -- so any subsequent version, including a hypothetical 3.0 would pass.