More AppVeyor tinkering to get hdbscan installed

The re-install hopefully upgrades versions
tox-dev/tox#791

Author: uellue

appveyor.yml

@@ -15,7 +15,7 @@ init:
   - "ECHO %PYTHON% %PYTHON_VERSION% %PYTHON_ARCH%"
 
 install:
-  - "%PYTHON%/Scripts/pip.exe install tox codecov cython"
+  - "%PYTHON%/Scripts/pip.exe install --force-reinstall tox codecov"
 
 test_script:
   - "%PYTHON%/Scripts/tox"