[R] Silencing the output of install.packages()

[Craig Citro]

> I think this has already been done in r59493ff of the svn repos by the
> indefatigable Prof. Ripley
>

Fantastic. I guess it *is* true: the first step in writing any patch
for R is to first ask yourself if Brian Ripley has already made the
change since the last release.

Thanks Brian!

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