Make (org-version) always returns a string, use it in exporters.
* org.el (org-version): When called non-interactively, insert
the short version string, otherwise send a message with the
complete version string.
* org-odt.el (org-odt-update-meta-file): Use (org-version) and
delegate checking whether `org-version' is known as a variable
there.
* org-html.el (org-export-as-html): Use (org-version).
* org-docbook.el (org-export-as-docbook): Ditto.
* org-latex.el (org-export-latex-make-header): Ditto.