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.