Docker build issues on Travis

[palfrey]

Seeing this for various builds on #1130 and #1146 and probably some other PRs. Errors vary, but some examples are:

• https://travis-ci.org/rust-lang/libc/jobs/459072481

Step 1/4 : FROM ubuntu:18.04
error parsing HTTP 404 response body: invalid character 'p' after top-level value: "404 page not found\n"

• https://travis-ci.org/rust-lang/libc/jobs/459075930

Step 1/14 : FROM ubuntu:16.04
unauthorized: authentication required

• https://travis-ci.org/rust-lang/libc/jobs/459075937

Step 1/5 : FROM ubuntu:17.10
17.10: Pulling from library/ubuntu
age configuration: unknown blob

None of them AFAIK have anything to do with the PRs in question, but are making testing/merging much harder.

[gnzlbg]

Sometimes CI times out for a variety of reasons. I don't know if there is something that we can do about this specific error.

In the meantime, I've retryed the cited PRs.

[palfrey]

I'm pretty sure this is actually just something wrong with Travis, and so I've finally given in and emailed their support email :)

[gnzlbg]

cc @kennytm

[palfrey]

Build is improved (at least for #1130), but still slightly off. https://travis-ci.org/rust-lang/libc/jobs/459715534 is the only failure of the last one, and that's a curl: (28) Operation timed out after 300808 milliseconds with 0 out of 0 bytes received which might eventually be worth using a travis_retry on