Fix the Travis deployment condition

See #2104 for details.

For now I'm hardcoding `$MONGODB = 3.4.17` just to get a release out there,
but we should probably use the globals going forward. Will do that in
a follow-up commit once I get the `travis-conditions` gem up and running and
hence can test `.travis.yml` changes without deploying.

Author: wojcikstefan

.travis.yml

@@ -102,5 +102,5 @@ deploy:
   on:
     tags: true
     repo: MongoEngine/mongoengine
-    condition: ($PYMONGO = 3.x) && ($MONGODB = 3.4)
+    condition: ($PYMONGO = 3.x) && ($MONGODB = 3.4.17)
     python: 2.7