Add a dummy KeyError to 1.8 so that src koans will run.

src/edgecase.rb

@@ -24,6 +24,11 @@ def in_ruby_version(*versions)
   yield if versions.any? { |v| ruby_version?(v) }
 end
 
+in_ruby_version("1.8") do
+  class KeyError < StandardError
+  end
+end
+
 # Standard, generic replacement value.
 # If value19 is given, it is used in place of value for Ruby 1.9.
 def __(value="FILL ME IN", value19=:mu)