Test memcached on Travis.
Only build libmemcached for deploys.
`go.net` has moved.

Ref. https://golang.org/s/go14subrepo. Update envconf and go-etcd.
Merge pull request #126 from mozilla-services/feature/travis-memcached

Run memcached tests on Travis
Disconnect duplicate clients.

Per @nikhilm's request.