Update libssn3 via .travis.yml

References #33.

.travis.yml

@@ -2,6 +2,11 @@ language: node_js
 node_js:
   - "7"
 dist: trusty
+addons:
+  apt:
+    packages:
+      # This is required to run new chrome on old trusty
+      - libnss3
 env:
 cache:
   yarn: true