Fix travis for docker image change

.travis.yml

@@ -2,10 +2,10 @@ language: python
 python:
   - "3.7"
 install:
-  - pip install -r requirements.txt
+  - pip install sphinx==2.2.0
   - pip install black codecov coverage
 script:
   - black --check entrypoint.py sphinx_action tests
   - coverage run --omit="*/tests*" -m unittest
 after_success:
-  - codecov
+  - codecov