Fix error return code on Ruby 1.8. (#338)

--before-install (and its brethren) were not causing fpm to exit on a
non-existant file error when using Ruby 1.8.

The root of the problem appears to be that that particular error-checking is
done inside a proc (lib/fpm/command.rb#L374); in Ruby 1.9, the return value of
the proc is used to determine whether `#call` was successful, while 1.8
gleefully ignores it.

    [$]> cat test.rb
    test = proc do
        return 1
    end

    test.call

    [$]> ruby-1.8.7-p334 test.rb; echo $?
    0

    [$]> ruby-1.9.3-p125 test.rb; echo $?
    test.rb:2:in `block in <main>': unexpected return (LocalJumpError)
        from test.rb:5:in `call'
        from test.rb:5:in `<main>'
    1

In this instance, we can use `exit` instead of `return`, and that'll be fine.