Travis download/install of clang-3.8 dependencies is failing

[actual-nh]

Describe the bug

On multiple PRs, Travis is failing (after about 30 seconds) due to a problem with downloading/installing clang-3.8 dependencies.

Steps To Reproduce

See #48054, #47562, #47653, and others.

Expected behavior

Actually compiling and running the tests.

Screenshots

Reading package lists...
Building dependency tree...
Reading state information...
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
 clang-3.8 : Depends: libclang1-3.8 (= 1:3.8-2ubuntu4) but it is not going to be installed
             Depends: libobjc-5-dev but it is not going to be installed
E: Unable to correct problems, you have held broken packages.

Versions and configuration

Travis clang-3.8 build.

[actual-nh]

Looks like it's doing better now.