Test, fix, and update documentation on Ruby versions supported

We claim to support Ruby 1.9.3/2.0.0+, which is clearly wrong. For instance, the `BigNumber` has no method of `bit_length` in [version 2.0](https://docs.ruby-lang.org/en/2.0.0/Bignum.html), and the method appears since [version 2.1](https://docs.ruby-lang.org/en/2.1.0/Bignum.html#method-i-bit_length).

A.J. pointed that "since 2.4 and earlier are apparently EOL, so it’s probably reasonable to not support them".

BTW, I did some testing and found that 2.4 works fine. However, when switching to 2.6, I found tuple encoding becomes different and failed the binding test. We need to fix 2.6 issues and probably test for 2.5, 2.7 as well.