old clang not available on Precise anymore

[JohanMabille]

I think it is related to #4327, we have been experiencing issues for the last couple of hours, "Disallowing sources: llvm-toolchain-precise-3.6" as shown in this log for instance.

The same issue occurs for versions 3.7, 3.8 and 3.9.

[JohanMabille]

I just found the commit removing all keys for precise environment, so I guess this is actually not a bug.