Looks like launchpad.net is having a problem.
https://travis-ci.org/travis-ci/php-src-builder/jobs/487462951#L3230
+wget https://launchpad.net/libmemcached/1.0/1.0.18/+download/libmemcached-1.0.18.tar.gz
--2019-02-01 15:20:06-- https://launchpad.net/libmemcached/1.0/1.0.18/+download/libmemcached-1.0.18.tar.gz
Resolving launchpad.net (launchpad.net)... 91.189.89.222, 91.189.89.223, 2001:67c:1560:8003::8004, ...
Connecting to launchpad.net (launchpad.net)|91.189.89.222|:443... connected.
HTTP request sent, awaiting response... 303 See Other
Location: https://launchpadlibrarian.net/165454254/libmemcached-1.0.18.tar.gz [following]
--2019-02-01 15:20:07-- https://launchpadlibrarian.net/165454254/libmemcached-1.0.18.tar.gz
Resolving launchpadlibrarian.net (launchpadlibrarian.net)... 91.189.89.228, 91.189.89.229, 2001:67c:1560:8003::8007, ...
Connecting to launchpadlibrarian.net (launchpadlibrarian.net)|91.189.89.228|:443... connected.
HTTP request sent, awaiting response... ..502 Proxy Error
2019-02-01 15:21:07 ERROR 502: Proxy Error.