Pin pip in CI for now

.appveyor.yml

@@ -15,7 +15,7 @@ install:
 
   # Upgrade to the latest version of pip to avoid it displaying warnings
   # about it being out of date.
-  - "python -m pip install --disable-pip-version-check --user --upgrade pip"
+  - "python -m pip install --disable-pip-version-check --user pip==18.1"
 
   # Installing Poetry
   - "python get-poetry.py --preview -y"

.travis.yml

@@ -11,7 +11,7 @@ cache:
     - "$HOME/.cache/pre-commit"
 
 install:
-  - pip install pip -U
+  - pip install pip==18.1
   - python get-poetry.py --preview -y
   - source $HOME/.poetry/env
   - poetry install -v