update Rakefile to use pre-defined directories

Rakefile

@@ -88,14 +88,14 @@ end
 task :default => :walk_the_path
 
 task :walk_the_path do
-  cd 'koans'
+  cd PROB_DIR
   ruby 'path_to_enlightenment.rb'
 end
 
 if defined?(Rake::RDocTask)
   Rake::RDocTask.new do |rd|
     rd.main = "README.rdoc"
-    rd.rdoc_files.include("README.rdoc", "koans/*.rb")
+    rd.rdoc_files.include("README.rdoc", "${PROB_DIR}/*.rb")
   end
 end
 
@@ -140,7 +140,7 @@ end
 
 task :run do
   puts 'koans'
-  Dir.chdir("src") do
+  Dir.chdir("${SRC_DIR}") do
     puts "in #{Dir.pwd}"
     sh "ruby path_to_enlightenment.rb"
   end