Fix makeebooks

fix below error.

    $ ./makeebooks ja
    using .mobi (you can change it via FORMAT environment variable. try 'mobi' or 'epub')
    convert content for 'ja' language
    processing /progit/ja/01-introduction/01-chapter1.markdown
    processing /progit/ja/02-git-basics/01-chapter2.markdown
    processing /progit/ja/03-git-branching/01-chapter3.markdown
    processing /progit/ja/04-git-server/01-chapter4.markdown
    processing /progit/ja/05-distributed-git/01-chapter5.markdown
    processing /progit/ja/06-git-tools/01-chapter6.markdown
    processing /progit/ja/07-customizing-git/01-chapter7.markdown
    processing /progit/ja/08-git-and-other-scms/01-chapter8.markdown
    processing /progit/ja/09-git-internals/01-chapter9.markdown
    ./makeebooks:73:in `system': can't convert nil into String (TypeError)
            from ./makeebooks:73
            from ./makeebooks:27:in `each'
            from ./makeebooks:27