[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)
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.