DATAMONGO-1699 - Upgrade travis-ci build to use MongoDB 3.4 server.

We now do it explicitly as there seems to be almost no movement getting the alias on the whitelist.

.travis.yml

@@ -16,7 +16,9 @@ env:
 addons:
   apt:
     sources:
-    - mongodb-3.4-precise
+      - mongodb-upstart
+      - sourceline: 'deb [arch=amd64] http://repo.mongodb.org/apt/ubuntu precise/mongodb-org/3.4 multiverse'
+        key_url: 'https://www.mongodb.org/static/pgp/server-3.4.asc'
     packages:
     - mongodb-org-server
     - mongodb-org-shell