Enforce isort in CI.

We need https://github.com/timothycrosley/isort/pull/1000 but there's no
release with this fix yet, so we'll install from GitHub.

.circleci/config.yml

@@ -90,12 +90,15 @@ jobs:
         working_directory: ~/transformers
         docker:
             - image: circleci/python:3.6
-        resource_class: small
+        resource_class: medium
         parallelism: 1
         steps:
             - checkout
-            - run: sudo pip install black
+            - run: sudo pip install --editable .
+            - run: sudo pip install torch tensorflow tensorboardX scikit-learn
+            - run: sudo pip install black git+git://github.com/timothycrosley/isort.git@e63ae06ec7d70b06df9e528357650281a3d3ec22#egg=isort
             - run: black --check --line-length 119 examples templates transformers utils
+            - run: isort --check-only --recursive examples templates transformers utils
     check_repository_consistency:
         working_directory: ~/transformers
         docker: