Drop the deb-src for clang.

The llvm.org/apt repo currently has an issue with the source packages,
and we don't need it on Travis anyways.
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.