(PPS-697) re-enable poetry caching

.travis.yml

@@ -31,7 +31,7 @@ before_install:
 install:
   - pip install --upgrade pip
   - pip install poetry
-  - poetry install --all-extras -vv --no-interaction --no-cache
+  - poetry install --all-extras -vv --no-interaction
   - poetry show -vv
   - psql -c 'SELECT version();' -U postgres
   - psql -U postgres -c "create database fence_test_tmp"