Pin pip and pipenv versions for now

pipenv is broken by:

pypa/pipenv#2956

.travis.yml

@@ -6,7 +6,8 @@ cache:
   - $HOME/.cache/pip
   - .arca/cache
 install:
-- python -m pip install pipenv --upgrade
+- python -m pip install --upgrade 'pip<18.1' 'pipenv<2018.10.7'
+- pipenv install 'git+https://github.com/pyvec/arca.git@pip_pipenv#egg=arca' 'docker~=3.2.1'
 - pipenv install --deploy --dev
 before_script:  # encrypted variables aren't accessible in PRs
 - 'if [ -n "$DOCKER_HUB_PASSWORD" ]; then docker login -u "$DOCKER_HUB_USERNAME" -p "$DOCKER_HUB_PASSWORD"; fi'