[RFC] Update clang version.

The build error discussed the end of #11 also occurred locally, but didn't happen when using another repo (there are a few available from http://llvm.org/apt).

I took this opportunity to update clang to 3.5, although that didn't change the number of warnings. The additional `ubuntu-toolchain-r` PPA was added because "gcc 4.8 backport (ppa) is necessary on Precise (for libstdc++)" (http://llvm.org/apt/).

[Build](https://travis-ci.org/fwalch/bot-ci/builds/35556187), [commit](https://github.com/fwalch/doc/commit/83b0fe50c57d5c73f00cc0a5ee87fa6a27d9d072), [![Clang Scan Build](https://fwalch.github.io/doc/reports/clang/badge.svg)](https://fwalch.github.io/doc/reports/clang)
The clang package installs only `/usr/bin/clang-3.5`, not `/usr/bin/clang`. I guess this is the difference between packages from the repo we used before (w/o version suffix) and the one this PR uses.

Symlinking a binary to an existing directory will create the symlink within this dir, e.g. `/home/travis/build/bin/gcc -> /usr/bin/gcc`.

"Justin M. Keyes" <notifications@github.com> wrote:

>In ci/common/dependencies.sh:
>
>> wget -q -O - http://llvm.org/apt/llvm-snapshot.gpg.key | sudo apt-key add - > sudo apt-get update -qq > sudo apt-get install -y -q clang-${CLANG_VERSION} > > - ln -fs /usr/bin/clang ${DEPS_BIN_DIR} > - ln -fs /usr/bin/scan-build ${DEPS_BIN_DIR} > + ln -fs /usr/bin/clang-${CLANG_VERSION} ${DEPS_BIN_DIR}/clang > + ln -fs /usr/bin/scan-build-${CLANG_VERSION} ${DEPS_BIN_DIR}/scan-build 
>
>Why is this now required? 
>
>Elsewhere we are still doing this:
>
>ln -fs ${DEPS_INSTALL_DIR}/doxygen/bin/doxygen ${DEPS_BIN_DIR} ... ln -fs /usr/bin/gcc ${DEPS_BIN_DIR} ln -fs /usr/bin/g++ ${DEPS_BIN_DIR} 
>
>Are those correct? It seems weird to symlink a binary to a directory, I'm guessing this is a ln quirk I am not aware of.
>
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