flask-mongoengine: PR's for expanded CI, updated PyMongo support

[Tony N]

Hello MongoEngine developers,

Today I published two new pull requests for review.

1. Upgrade pymongo tracking for pymongo 3.6.0+

    https://github.com/MongoEngine/flask-mongoengine/pull/346

    This addresses issues in https://github.com/MongoEngine/flask-mongoengine/issues/325 for the moment, however

    there is larger issue to tackle of us relying on a private API in pymongo that isn't stable.

    I am trying to see if we can have this merged and published as a solution for current Flask-DebugToolbar users, so

    they can use the latest pymongo and still use the flask-debugtoolbar panel provided.

2. Refreshed Travis CI and packaging in sync with MongoEngine 0.15.3

    https://github.com/MongoEngine/flask-mongoengine/pull/347

    This approach adds support for pymongo versions to the matrix. The reason why is to gain additional

    coverage where pymongo API changes could introduce breaking changes we can handle earlier

    on (hopefully!).

Please let me know if you have any feedback. Have a good weekend.

Best,

Tony Narlock