Updated version with working Calibre's ebook-convert tool

-  fixed issue that the execution of Calibre's `ebook-convert` didn't…

… worked correct,

   therefore changed the internal Calibre path from `/opt/calibre/bin` to `/opt/calibre`;
   thanks to @bodybybuddha and @adocampo for testing and finding a solution for this issue;
   see for details at issue #28