[WIP/IGNORE] Make it easy to disable the use of clang 3.4.

It appears the llvm.org/apt repository isn't always reliable.  So let's
make using it conditional, just in case they're failing for some reason.

Note: disabling the use of 3.4 means that we don't get leak detection.
But, we get a more stable build platform.

**Ignore this for now while I test a few things with Travis CI**