Try a different deployment condition

The previous one was a verbatim copy-paste of what TravisCI's Support suggested
to me, but sadly it didn't work. See
https://travis-ci.org/MongoEngine/mongoengine/jobs/544655132. That build
should've triggered a deployment.

This time I'm trying a different syntax, primarily influenced by
https://docs.travis-ci.com/user/conditions-v1#boolean-operators.

Author: wojcikstefan

.travis.yml

@@ -95,5 +95,5 @@ deploy:
   on:
     tags: true
     repo: MongoEngine/mongoengine
-    condition: ($PYMONGO = 3.x) AND ($MONGODB = 3.4)
+    condition: $PYMONGO = 3.x AND $MONGODB = 3.4
     python: 2.7