add pip freeze to travis.yml

and stop building docs on travis, we are already building them on circle

.travis.yml

@@ -12,6 +12,7 @@ install:
 - pip install -r dev-requirements.txt
 - python setup.py bdist_wheel
 - pip install dist/*.whl
+- pip freeze
 script:
 # cd into tests so CWD being repo2docker does not hide
 # possible issues with MANIFEST.in
@@ -22,10 +23,6 @@ script:
     travis_retry pytest --durations 10 --cov repo2docker -v ${REPO_TYPE} || exit 1;
   fi;
   popd;
-- pip install -r docs/doc-requirements.txt
-- pushd docs;
-  make html || exit 1;
-  popd;
 after_success:
 - pip install codecov
 - pushd tests && codecov && popd