Simplify readme and requirements

Significantly simplify readme and related requirements:

* Do not require Rails (I do not believe there is a Rails app within this repository's source code)
* Do not require RHEL (there is no reason RHEL is required, any system that supports Ruby will suffice)
* Include installation instructions for non-RHEL systems
* Use Bundler to manage dependencies (and take advantage of the already-included Gemfile)
* Include directions to install Bundler (it is currently referenced, but no installation instructions are provided)
* Do not list Jekyll dependencies (Bundler will install these automatically)
* No need to update Gems immediately after installation, Gemfile will require latest version
* Do not require `ruby-devel` or `sqlite` to run Jekyll
* Link out to supporting documentation
* Remove reference to uncomment `therubyracer` Gem which is not in Gemfile (and is not required to run Jekyll)
* Do not install Jekyll development dependencies
* Recommend cloning via Git to encourage contribution

[Rendered view](https://github.com/benbalter/HealthCare.gov-Open-Source-Release/blob/readme-cleanup/README.md)