Beamer template: add booktabs package

Since 1.12.2 the LaTeX writer uses booktabss \toprule, \bottomrule etc. in tables, which requires an additional \usepackage (that default.latex already got with 93d667c0be8f7ad48f85e1144de922cd48a7d757)

This fixes the following issue:

````
echo -e "------\nabc\n------" | pandoc -t latex -o foo.pdf # works fine

echo -e "------\nabc\n------" | pandoc -t beamer -o foo.pdf
pandoc: Error producing PDF from TeX source.
! Undefined control sequence.
\beamer@doifinframe ...able}[c]{@{}l@{}} \toprule 
                                                  \addlinespace abc \\\addli...
l.57 \end{frame}
````