Update mongoengine to 0.18.2

[pyup-bot]

This PR updates mongoengine from 0.16.3 to 0.18.2.

Changelog

0.18.2

=================
- Replace some of the deprecated PyMongo v2.x methods with their v3.x equivalents 2097
- Various code clarity and documentation improvements

0.18.1

=================
- Fix a bug introduced in 0.18.0 which was causing `.save()` to update all the fields
 instead of updating only the modified fields. This bug only occurs when using custom pk 2082
- Add Python 3.7 in travis 2058

0.18.0

=================
- Drop support for EOL'd MongoDB v2.6, v3.0, and v3.2.
- MongoEngine now requires PyMongo >= v3.4. Travis CI now tests against MongoDB v3.4 – v3.6 and PyMongo v3.4 – v3.6 (2017 2066).
- Improve performance by avoiding a call to `to_mongo` in `Document.save()` 2049
- Connection/disconnection improvements:
 - Expose `mongoengine.connection.disconnect` and `mongoengine.connection.disconnect_all`
 - Fix disconnecting 566 1599 605 607 1213 565
 - Improve documentation of `connect`/`disconnect`
 - Fix issue when using multiple connections to the same mongo with different credentials 2047
 - `connect` fails immediately when db name contains invalid characters 2031 1718
- Fix the default write concern of `Document.save` that was overwriting the connection write concern 568
- Fix querying on `List(EmbeddedDocument)` subclasses fields 1961 1492
- Fix querying on `(Generic)EmbeddedDocument` subclasses fields 475
- Fix `QuerySet.aggregate` so that it takes limit and skip value into account 2029
- Generate unique indices for `SortedListField` and `EmbeddedDocumentListFields` 2020
- BREAKING CHANGE: Changed the behavior of a custom field validator (i.e `validation` parameter of a `Field`). It is now expected to raise a `ValidationError` instead of returning True/False 2050
- BREAKING CHANGES (associated with connect/disconnect fixes):
 - Calling `connect` 2 times with the same alias and different parameter will raise an error (should call `disconnect` first).
 - `disconnect` now clears `mongoengine.connection._connection_settings`.
 - `disconnect` now clears the cached attribute `Document._collection`.
- BREAKING CHANGE: `EmbeddedDocument.save` & `.reload` is no longier exist 1552

0.17.0

=================
- Fix .only() working improperly after using .count() of the same instance of QuerySet
- Fix batch_size that was not copied when cloning a queryset object 2011
- POTENTIAL BREAKING CHANGE: All result fields are now passed, including internal fields (_cls, _id) when using `QuerySet.as_pymongo` 1976
- Document a BREAKING CHANGE introduced in 0.15.3 and not reported at that time (1995)
- Fix InvalidStringData error when using modify on a BinaryField 1127
- DEPRECATION: `EmbeddedDocument.save` & `.reload` are marked as deprecated and will be removed in a next version of mongoengine 1552
- Fix test suite and CI to support MongoDB 3.4 1445
- Fix reference fields querying the database on each access if value contains orphan DBRefs

=================

Links

• PyPI: https://pypi.org/project/mongoengine
• Changelog: https://pyup.io/changelogs/mongoengine/
• Repo: https://github.com/MongoEngine/mongoengine/tarball/master
• Homepage: http://mongoengine.org/

[pyup-bot]

Closing this in favor of #527