Add `gem.required_ruby_version = ">= 2.0.0"` explicitly
Add `gem.required_ruby_version = ">= 2.0.0"` explicitly

I had used `String#+@` 48bcac2e in ISUCON10, but it requires Ruby 2.3+.

Since `mysql2` gem supports Ruby 2.0, so I've added the
`required_ruby_version` explicitly to ease to find the supported
version.

tagomoris/mysql2-cs-bind