[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)