Merge pull request #641 from iij/pr-minirake-exit-status

minirake exit status not 0, when build failed.

minirake

@@ -411,6 +411,7 @@ class RakeApp
       else
         puts ex.backtrace.find {|str| str =~ /#{@rakefile}/ } || ""
       end
+      exit 1
     end    
   end
 end