Ubuntu package source status

Hi,

At Mapbox we have been using the llvm-toolchain-precise-3.5 package
source to install clang-3.5 on Travis CI VMs for testing purposes
(Travis VM images are Ubuntu 12.04). This has worked fairly well for
us; thanks for providing it. However, the 3.5 source seems to have
recently disappeared, breaking our builds. No problem, we thought,
we'll update to 3.6. However, this doesn't seem to work either. When
using either the llvm-toolchain-precise or llvm-toolchain-precise-3.6
sources, no clang-3.6 package can be found:

$ sudo -E apt-add-repository -y "deb http://llvm.org/apt/precise/
llvm-toolchain-precise main"
Installing APT Packages (BETA)
$ export DEBIAN_FRONTEND=noninteractive
$ sudo -E apt-get -yq update &>> ~/apt-get-update.log
$ sudo -E apt-get -yq --no-install-suggests --no-install-recommends
--force-yes install gdb clang-3.6 libstdc++-4.9-dev libstdc++6
libllvm3.4 xutils-dev libxxf86vm-dev x11proto-xf86vidmode-dev
mesa-utils
Reading package lists...
Building dependency tree...
Reading state information...
E: Unable to locate package clang-3.6

Full output here: Travis CI - Test and Deploy Your Code with Confidence
Similar output when using the llvm-toolchain-precise-3.6 source here:
Travis CI - Test and Deploy Your Code with Confidence

Are there packages missing from these sources? What version is the
unsuffixed llvm-toolchain-precise source expected to contain? And for
the future avoidance of unexpected build breakage, is there a setup we
can use where packages won't disappear from underneath us?

Thanks!

John