Fix warning on Django 3.1

Fix this warning raised from Django 3.1 onwards:

```
venv/lib/python3.8/site-packages/djgeojson/fields.py:60
  /.../venv/lib/python3.8/site-packages/djgeojson/fields.py:60: RemovedInDjango40Warning: django.utils.translation.ugettext_lazy() is deprecated in favor of django.utils.translation.gettext_lazy().
    description = _("Geometry as GeoJSON")
```

Also add Django 3.1 to the travis definition.

.travis.yml

@@ -27,6 +27,7 @@ env:
   - DJANGO_VERSION=2.1.15
   - DJANGO_VERSION=2.2.10
   - DJANGO_VERSION=3.0.3
+  - DJANGO_VERSION=3.1.2
 
 install:
    # This is a dependency of our Django test script
@@ -53,8 +54,12 @@ matrix:
           env: DJANGO_VERSION=2.2.10
         - python: 3.4
           env: DJANGO_VERSION=3.0.3
+        - python: 3.4
+          env: DJANGO_VERSION=3.1.2
         - python: 3.5
           env: DJANGO_VERSION=3.0.3
+        - python: 3.5
+          env: DJANGO_VERSION=3.1.2
         - python: 3.8
           env: DJANGO_VERSION=2.0.13
         - python: 3.8

djgeojson/fields.py

@@ -1,7 +1,7 @@
 from __future__ import unicode_literals
 
+import django
 from django.forms.widgets import HiddenInput
-from django.utils.translation import ugettext_lazy as _
 from django.core.exceptions import (ValidationError,
                                     ImproperlyConfigured)
 try:
@@ -22,6 +22,12 @@ def __init__(self, *args, **kwargs):
     JSONFormField = Missing
 
 
+if django.VERSION >= (3, 1):
+    from django.utils.translation import gettext_lazy as _
+else:
+    from django.utils.translation import ugettext_lazy as _
+
+
 class GeoJSONValidator(object):
     def __init__(self, geom_type):
         self.geom_type = geom_type