Try a different deployment condition

This time my attempt is based on the output found in another job that didn't
trigger a deployment: https://travis-ci.org/MongoEngine/mongoengine/jobs/544664203

```
/home/travis/.travis/job_stages: line 660: expected `)'
/home/travis/.travis/job_stages: line 660: syntax error near `AND'
/home/travis/.travis/job_stages: line 660: `  if [[ ($TRAVIS_REPO_SLUG = "MongoEngine/mongoengine") && ($TRAVIS_PYTHON_VERSION = 2.7) && ($PYMONGO = 3.x AND $MONGODB = 3.4) && ("$TRAVIS_TAG" != "") ]]; then'
```

See 80ca636 and
40ba51a for my previous attempts.

.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) && ($MONGODB = 3.4)
     python: 2.7