Use dev install of pip to fix coverage check

.travis.yml

@@ -27,7 +27,7 @@ install:
   - pip install pytest pytest-cov coveralls
   - pip install flake8 black
   - pip install mypy
-  - pip install .$DEP
+  - pip install -e .$DEP
 script:
   - pytest --cov=cgp
   - black --check .