Do not use preview versions of Poetry in CI

.github/workflows/main.yml

@@ -41,8 +41,7 @@ jobs:
       - name: Install poetry
       - name: Install poetry
         shell: bash
         shell: bash
         run: |
         run: |
-          curl -fsS -o get-poetry.py https://raw.githubusercontent.com/python-poetry/poetry/master/get-poetry.py
-          python get-poetry.py --preview -y
+          python get-poetry.py -y
           echo "::set-env name=PATH::$HOME/.poetry/bin:$PATH"
           echo "::set-env name=PATH::$HOME/.poetry/bin:$PATH"
 
 
       - name: Configure poetry
       - name: Configure poetry