[feature] list ruby language vulnerabilities

This is an attempt to implement #20 Feedback is certainly welcome, and needed on the format and file paths I've chosen.

Implementation:

I'm assuming that multiple platforms should be supported e.g. mri, jruby, rbx, and I've borrowed the format largely from the existing one for gems, replacing the `gem` attribute with `platform`

I've also changed the commonly used patch release syntax such as `1.9.3-p447` into `1.9.3.447` this is to make automated parsing by a tool similar to the way [bundler-audit](https://github.com/postmodern/bundler-audit) does for gems, that is, make use of `Gem::Version` (rubygems >= 1.8.0) which correctly handles comparisons using `~>` and other semantic operators on version strings with 4 numeric parts separated by periods.

I've also added almost all of the vulnerabilities listed on http://www.ruby-lang.org/en/security/ into [another feature branch](https://github.com/jeremyolliver/ruby-advisory-db/tree/feature/add-mri-vulnerabilities) which I can send a PR for if this format looks good - but I didn't want this pull request to get too big for now.