Fix CI poetry installation (#434)

appveyor.yml

@@ -32,7 +32,7 @@ install:
   # needed? to fix `appveyor` command
   - 'set PATH=C:\Program Files\AppVeyor\BuildAgent\;%PATH%'
 
-  - 'curl -sSL https://raw.githubusercontent.com/sdispater/poetry/master/get-poetry.py | %py%'
+  - 'curl -sSL https://install.python-poetry.org | %py% -'
   - 'set PATH=%USERPROFILE%\.poetry\bin;%PATH%'
 
   # Prevents: "The current Python version (2.7.15) is not supported by the project (^3.6)"