Let DjangoInstalledChecker warn when Django isn't available

by ignoring the import failure Python will not produce a traceback
but instead let DjangoInstalledChecker do its job and warn the user.

I also add a CI build stage to test this scenario since we don't want
to continue if this isn't working.

Finally properly fixes #96.

.travis.yml

@@ -7,14 +7,18 @@ python:
 env:
   - DJANGO=1.11
   - DJANGO=2.0
+stages:
+  - django_not_installed
+  - test
 matrix:
   exclude:
     # Python/Django combinations that aren't officially supported
     - { python: 2.7, env: DJANGO=2.0 }
   include:
-    - { python: 3.6, env: TOXENV=flake8 }
-    - { python: 3.6, env: TOXENV=pylint }
-    - { python: 3.6, env: TOXENV=readme }
+    - { stage: django_not_installed, python: 3.6, env: TOXENV=django_not_installed }
+    - { stage: test, python: 3.6, env: TOXENV=flake8 }
+    - { stage: test, python: 3.6, env: TOXENV=pylint }
+    - { stage: test, python: 3.6, env: TOXENV=readme }
   allow_failures:
     - env: TOXENV=flake8
     - env: TOXENV=pylint

pylint_django/plugin.py

@@ -2,7 +2,6 @@
 from pylint.checkers.base import NameChecker
 from pylint_plugin_utils import get_checker
 
-from pylint_django.augmentations import apply_augmentations
 from pylint_django.checkers import register_checkers
 
 # we want to import the transforms to make sure they get added to the astroid manager,
@@ -26,4 +25,10 @@ def register(linter):
     register_checkers(linter)
 
     # register any checking fiddlers
-    apply_augmentations(linter)
+    try:
+        from pylint_django.augmentations import apply_augmentations
+        apply_augmentations(linter)
+    except ModuleNotFoundError:
+        # probably trying to execute pylint_django when Django isn't installed
+        # in this case the django-not-installed checker will kick-in
+        pass

tox.ini

@@ -3,6 +3,7 @@
 
 [tox]
 envlist =
+    django_not_installed
     flake8
     pylint
     readme
@@ -11,6 +12,7 @@ envlist =
 
 [testenv]
 commands =
+    django_not_installed: pylint --load-plugins=pylint_django ./setup.py | grep django-not-available
     flake8: flake8
     pylint: pylint --rcfile=tox.ini pylint_django setup
     readme: python setup.py check --restructuredtext --strict