Switch Walrat to a submodule at vendor/walrat

This will make for easier development (ie. without having to push Walrat
changes to a remote before testing them locally with Walrus).
Silence Gemfile warnings
Prepare to run under Ruby 2.1.3

The array change is necessary because the `#to_s` behavior on `Array`
changed from 1.8.7:

  `['foo', 'bar']` -> "foobar"

to 2.1.3:

  `['foo', 'bar']` -> "[\"foo\", \"bar\"]"

With this commit, the spec suite is green under 2.1.3.
Simplify RawText implementation

These are equivalent, because `Node#to_s` returns the lexeme that was
passed to `Node#initialize`, with `#to_s` already applied to it.