Detect possible RAM by Ruby and just use `ulimit` command

- Right now the bin script is not getting executed because its getting
  confused by the path.
- Found out this while upgrading codetriage that while running rake
  tasks it gives error:

  Errno::ENOENT: No such file or directory - bin/heroku_ulimit_to_ram

- It is not able to resolve the path to the script.
- This fix will only use `ulimit` command and perform rest of the job in Ruby.