Protect against symbol injection DoS attacks

Expands the YAML whitelisting patch to include whitelisting symbols.

The five symbols whitelisted are surveyed from all metadata of all existing gems using [this gist](https://gist.github.com/4695129):

 * `:development` and `:runtime`, used legitimately by `Gem::Dependency`
 * `:rdoc` which is incorrectly serialized in rdoc_options in "fisheye-crucible-0.0.1"
 * `:json` and `:mocha`, which are incorrectly serialized gem names from `Gem::Dependency`s in some versions of "ezri" (0.03 - 0.07)

There are four different ways to serialize symbols in YAML:

 * `:symbol` parsed as a scalar
 * `:"symbol"` parsed as a scalar
 * `!ruby/sym symbol` parsed as a tagged value (also subclasses like `!ruby/sym:MySymbolSubclass symbol`)
 * `!ruby/symbol symbol` parsed as a tagged value (also subclasses like `!ruby/symbol:MySymbolSubclass symbol`)