Streamline the travis deploy builds; when TRAVIS_TAG is set
we'll run `--release` builds.
Don't error out in fontconfig/build.rs if fontconfig is not
installed. This makes it possible to `cargo test --all` again
on the mac at the cost of potentially making it harder to troubleshoot
problems with not having fontconfig installed on linux.
However: the get-deps script is responsible for installing that.
