Add libnss to the travis apt and macOS packages
The travis image already has these packages:
libnss3:amd64 2:3.28.4-0ubuntu0.14.04.3
libnss3-dev:amd64 2:3.28.4-0ubuntu0.14.04.3
libnss3-nssdb 2:3.28.4-0ubuntu0.14.04.3
But we should add them to the travis config, in case they are removed in a future version.