Revert snap MongoDB from v3.2.18 back to v3.2.16 because of SSL cert error. Trying to fix snap build.

snapcraft.yaml

@@ -54,7 +54,7 @@ apps:
 
 parts:
     mongodb:
-        source: https://fastdl.mongodb.org/linux/mongodb-linux-x86_64-ubuntu1604-3.2.18.tgz
+        source: https://fastdl.mongodb.org/linux/mongodb-linux-x86_64-ubuntu1604-3.2.16.tgz
         plugin: dump
         stage-packages: [libssl1.0.0]
         filesets: