Also catch NameError exceptions in read_env

The django.conf.settings module could be successfully imported but the
BASE_DIR constant could not be defined, which would raise a NameError.

This small edit ensures that we're catching both potential failures.

environ/environ.py

@@ -741,7 +741,7 @@ def read_env(cls, env_file=None, **overrides):
             try:
                 from django.conf import settings
                 env_file = os.path.join(settings.BASE_DIR, '.env')
-            except ImportError:
+            except (ImportError, NameError):
                 logger.info(
                     "%s doesn't exist - if you're not configuring your "
                     "environment separately, create one." % env_file)