2015-08-31: Release version 3.17.91

    * New and updated translations
      - el, courtesy of Tom Tryfonidis
      - es, courtesy of Daniel Mustieles
      - he, courtesy of Yosef Or Boczko
      - pt, courtesy of Pedro Albuquerque
    * Fixed about dialog not showing close button (Olav Vitters)
    * Website on about dialog should not be translatable (Piotr DrÄ…g)