Documentation fixes for markdown intra-word italics and adding ruby markdown blocks

I noticed a few pages of documentation displayed poorly because of markdown interpreting underscores to signify italics. This PR fixes those issues. Some examples:

  * https://www.relishapp.com/rspec/rspec-expectations/v/2-12/docs/built-in-matchers/end-with-matcher
  * https://www.relishapp.com/rspec/rspec-expectations/v/2-12/docs/built-in-matchers/start-with-matcher
  * https://www.relishapp.com/rspec/rspec-expectations/v/2-12/docs/built-in-matchers/specify-types-of-objects

While I was in there I also noticed the opportunity to normalize a lot of the code blocks to explicitly specify ruby as the language, which fixes some display issues. Some (not all) examples of format weirdness:

  * https://www.relishapp.com/rspec/rspec-expectations/v/2-12/docs/built-in-matchers/exist-matcher
  * https://www.relishapp.com/rspec/rspec-expectations/v/2-12/docs/built-in-matchers/raise-error-matcher
  * https://www.relishapp.com/rspec/rspec-expectations/v/2-12/docs/built-in-matchers/have-n-items-matcher