Use older version of poetry since current one is buggy

Cito · Cito · commit a29107465800 · 2019-05-01T00:38:19.000+02:00
see python-poetry/poetry#1060
diff --git a/.travis.yml b/.travis.yml
@@ -8,7 +8,7 @@ python:
     - 3.7
 
 install:
-    - pip install poetry
+    - pip install "poetry==0.12.11"
     - poetry install
 
 script:
