Try to upgrade pip and install poetry before using poetry on Travis.

netsettler · netsettler · commit 5cebaa3fa2da · 2020-03-12T02:23:10.000-04:00
diff --git a/.travis.yml b/.travis.yml
@@ -3,6 +3,8 @@ sudo: false
 python:
 - '3.6'
 install:
+- pip install --upgrade pip==19.0.3
+- pip install poetry
 - poetry install
 - pip install coveralls
 script:
