Fix #415 (reltool vsn check)

* Refactor reltool version check to use vsn app key if ebin/reltool.app
  exists.

  OTP releases prior to R14 didn't install the app file, so we still
  have to support getting the version string from the lib_dir path. If
  you're using an R13B03 or R13B04 install that doesn't have version
  strings in lib_dir paths (lib/reltool vs lib/reltool-0.5.3), we won't
  be able to retrieve the version string. Given R13's age and that it's
  unusual to have vsn-less lib_dir paths, this shouldn't be a problem.

* While at it, delete trailing white space.