Use racc 1.4.15

Author: daemonsy

Rakefile

@@ -62,7 +62,7 @@ ERR
   end
 
   assert_dependency_version("Ragel", "7.0.0.9", "ragel -v")
-  assert_dependency_version("Racc", "1.4.14", %|ruby -e "require 'racc'; puts Racc::VERSION"|)
+  assert_dependency_version("Racc", "1.4.15", %|ruby -e "require 'racc'; puts Racc::VERSION"|)
 
   `rm -f lib/graphql/language/parser.rb lib/graphql/language/lexer.rb `
   `racc lib/graphql/language/parser.y -o lib/graphql/language/parser.rb`