Travis build failures on Linux during September 2019

[asomers]

Beginning a few days ago all of our Linux builds on Travis began to fail. It seems like Travis changed the contents of their image, but didn't mention it on their blog or anywhere else. We're seeing the following failures:

1. sys::test_socket::test_af_alg_aead returns ENOPROTOOPT
2. sys::test_socket::test_af_alg_cipher returns ENOPROTOOPT
3. sys::test_uio::test_process_vm_readv returns ENOSYS
4. sys::test_wait::ptrace::test_wait_ptrace fails, apparently because SIGTRAP wasn't caught.

[asomers]

Switching the build image to Ubuntu 18.04 didn't help. I've raised the issue on Travis's forum here:
https://travis-ci.community/t/unannounced-change-to-linux-environment-on-sep-6/5075

[asomers]

It seems that Travis enabled Seccomp in their containers on about 6-September, then over the course of 12 days changed the config once or twice, and at one point completely disabled it. There were no notifications on Travis's blob or anywhere else. PR #1123 fixed the build, but it may break again if Travis changes something. See also https://travis-ci.community/t/unannounced-change-to-linux-environment-on-sep-6/5075/5