avoid Dir.exist? that is not available in Ruby1.8

tasks/mrbgem_spec.rake

@@ -55,7 +55,7 @@ module MRuby
         MRuby::Gem.current = self
         @build.compilers.each do |compiler|
           compiler.include_paths << "#{dir}/include"
-        end if Dir.exist? "#{dir}/include"
+        end if File.exist? "#{dir}/include"
         MRuby::Build::COMMANDS.each do |command|
           instance_variable_set("@#{command}", @build.send(command).clone)
         end

tasks/mruby_build.rake

@@ -194,7 +194,7 @@ module MRuby
     end
 
     def run_bintest
-      targets = @gems.select { |v| Dir.exists? "#{v.dir}/bintest" }.map { |v| filename v.dir }
+      targets = @gems.select { |v| File.exist? "#{v.dir}/bintest" }.map { |v| filename v.dir }
       sh "ruby test/bintest.rb #{targets.join ' '}"
     end