Use require_relative instead of require (#37)

Same update as whitequark/parser#1003.

Author: koic

lib/ast.rb

@@ -11,7 +11,7 @@
 # additional recommendations and design patterns.
 #
 module AST
-  require 'ast/node'
-  require 'ast/processor'
-  require 'ast/sexp'
+  require_relative 'ast/node'
+  require_relative 'ast/processor'
+  require_relative 'ast/sexp'
 end

lib/ast/processor.rb

@@ -6,7 +6,7 @@ module AST
   #
   # @deprecated Use {AST::Processor::Mixin} instead.
   class Processor
-    require 'ast/processor/mixin'
+    require_relative 'processor/mixin'
     include Mixin
   end
 end