Fix flake8 error and pin flake8 to 3.6.0 (#466)

Author: Ross Mechanic

.travis.yml

@@ -15,7 +15,7 @@ env:
 
 install:
   - pip install -U coverage codecov
-  - pip install -U flake8
+  - pip install -U flake8==3.6.0
   - pip install -U $DJANGO
   - pip freeze
 

simple_history/management/commands/populate_history.py

@@ -95,13 +95,17 @@ def _model_from_natural_key(self, natural_key):
             except LookupError:
                 model = None
         if not model:
-            raise ValueError(self.MODEL_NOT_FOUND +
-                             " < {model} >\n".format(model=natural_key))
+            msg = self.MODEL_NOT_FOUND + " < {model} >\n".format(
+                model=natural_key
+            )
+            raise ValueError(msg)
         try:
             history_model = utils.get_history_model_for_model(model)
         except NotHistoricalModelError:
-            raise ValueError(self.MODEL_NOT_HISTORICAL +
-                             " < {model} >\n".format(model=natural_key))
+            msg = self.MODEL_NOT_HISTORICAL + " < {model} >\n".format(
+                model=natural_key
+            )
+            raise ValueError(msg)
         return model, history_model
 
     def _bulk_history_create(self, model, batch_size):