Loading babel TeX package to avoid printing tilde in references

Surprisingly, this fixes the tilde introduced by texi2pdf for cross references and "et al.".

https://stat.ethz.ch/pipermail/r-devel/2012-January/063084.html

Linked from there, especially useful: http://www2.it.lut.fi/wiki/doku.php/common/latex_hints
Merge pull request #49 from yihui/vignette-tilde

Loading babel TeX package to avoid printing tilde in references