Update the test matrix to reflect what's supported in 2019 (#2066)

Previously, we were running the test suite for several combinations of MongoDB,
Python, and PyMongo:
- PyPy, MongoDB v2.6, PyMongo v3.x (which really means v3.6.1 at the moment)
- Python v2.7, MongoDB v2.6, PyMongo v3.x
- Python v3.5, MongoDB v2.6, PyMongo v3.x
- Python v3.6, MongoDB v2.6, PyMongo v3.x
- Python v2.7, MongoDB v3.0, PyMongo v3.5.0
- Python v3.6, MongoDB v3.0, PyMongo v3.5.0
- Python v3.5, MongoDB v3.2, PyMongo v3.x
- Python v3.6, MongoDB v3.2, PyMongo v3.x
- Python v3.6, MongoDB v3.4, PyMongo v3.x
- Python v3.6, MongoDB v3.6, PyMongo v3.x

There were a couple issues with this setup:
1. MongoDB v2.6 – v3.2 have reached their End of Life already (v2.6 almost 3
   years ago!). See the "MongoDB Server" section on
   https://www.mongodb.com/support-policy.
2. We were only testing two recent-ish PyMongo versions (v3.5.0 & v3.6.1).
   We were not testing the oldest actively supported MongoDB/PyMongo/Python
   setup.

This PR updates the test matrix so that these problems are solved. For the
sake of simplicity, it does not yet attempt to cover MongoDB v4.0:
- PyPy, MongoDB v3.4, PyMongo v3.x (aka v3.6.1 at the moment)
- Python v2.7, MongoDB v3.4, PyMongo v3.x
- Python v3.5, MongoDB v3.4, PyMongo v3.x
- Python v3.6, MongoDB v3.4, PyMongo v3.x
- Python v2.7, MongoDB v3.4, PyMongo v3.4
- Python v3.6, MongoDB v3.6, PyMongo v3.x

tests/queryset/modify.py

@@ -2,8 +2,6 @@ import unittest
 
 from mongoengine import connect, Document, IntField, StringField, ListField
 
-from tests.utils import requires_mongodb_gte_26
-
 __all__ = ("FindAndModifyTest",)
 
 
@@ -96,7 +94,6 @@ class FindAndModifyTest(unittest.TestCase):
         self.assertEqual(old_doc.to_mongo(), {"_id": 1})
         self.assertDbEqual([{"_id": 0, "value": 0}, {"_id": 1, "value": -1}])
 
-    @requires_mongodb_gte_26
     def test_modify_with_push(self):
         class BlogPost(Document):
             tags = ListField(StringField())