Error if --workdir flag points somewhere that doesn't exist or is not…

[jordansissel]

… a directory.

Fixes #1938

[jordansissel]

% bundle exec rspec -fd spec/fpm/command_spec.rb -e 'workdir'
Run options: include {:full_description=>/workdir/}

FPM::Command
  --workdir
    with an nonexistent path
      should fail aka return nonzero exit
    with a path that is not a directory
      should fail aka return nonzero exit

Finished in 0.0014 seconds (files took 0.14484 seconds to load)
2 examples, 0 failures

[jordansissel]

fpm 1.15.0 is released and contains this improvement.