Yesod updates for GHC 7.8

Same idea as https://github.com/TechEmpower/FrameworkBenchmarks/pull/832. Pinging @kazu-yamamoto.
If configuration files are too be updated prefer old if user modifications
exist otherwise prefer updated.
On Oct 6, 2014 9:13 AM, "Mike Smith" <notifications@github.com> wrote:

> In toolset/setup/linux/prerequisites.sh:
>
> >
> >  # WARNING: DONT PUT A SPACE AFTER ANY BACKSLASH OR APT WILL BREAK
> > -sudo apt-get -y install \
> > +sudo apt-get -y install -o Dpkg::Options::="--force-confdef" -o Dpkg::Options::="--force-confold" \
>
> Before merging, I would love to know what these extra flags do.
>
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> <https://github.com/TechEmpower/FrameworkBenchmarks/pull/834/files#r18458345>
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