[DO NOT MERGE] Replace syntax gem with coderay

Proof-of-concept PR for replacing syntax gem. Visual output does not change, but the underlying HTML does.

Updating fixtures is a PITA since the backtraces keep changing. See https://github.com/xaviershay/rspec-core/compare/regen-html-fixtures for how I did this. This is relevant if you want a nice diff of changes to review. If we don't particularly care then things become a lot easier.

To fix the build, still need to regenerate fixtures for:

* Rubinius 1.8.7
* Rubinius 1.9.3
* CRuby 1.8.7
* CRuby 1.9.2
* CRuby 2.0.0

These either didn't install cleanly on my crufty development laptop, or can't build nokogiri for whatever reason, and I got bored of trying to fix it.

Addresses #932.