Travis exits at the first failing test, pin codecov at v4.5.4 (#14179)

Before this commit we used to run the entire unit test suite
in the presence of a failure. Since we currently rely a lot
on the state of the filesystem etc. the end report was most
of the time showing spurious failures that were a consequence
of the first failing test.

This PR makes unit tests exit at the first failing test

Also, pin codecov at v4.5.4 (last one supporting Python 2.6)

.travis.yml

@@ -167,7 +167,7 @@ install:
   - pip install --upgrade pip
   - pip install --upgrade six
   - pip install --upgrade setuptools
-  - pip install --upgrade codecov
+  - pip install --upgrade codecov coverage==4.5.4
   - pip install --upgrade flake8
   - pip install --upgrade pep8-naming
   - if [[ "$TEST_SUITE" == "doc" ]]; then