Update biblatex support in style file.

* style/biblatex.el: Update copyright years and specify last
`biblatex' version supported.
(LaTeX-biblatex-executebibliographyoptions-options): Update.
(LaTeX-biblatex-language-list): Ditto.
(LaTeX-arg-biblatex-cites): Fix condition on `items' variable to
adapt to the `TeX-completing-read-multiple' return value change.