Use --force-reinstall when installing tox

This attempts to fix the recent failures on Windows, as
suggested by tox-dev/tox#791

Author: nicoddemus

.travis.yml

@@ -14,7 +14,8 @@ env:
     - PYTEST_ADDOPTS=-vv
 
 install:
-  - python -m pip install --upgrade --pre tox
+  - python -m pip install --upgrade pip
+  - python -m pip install --upgrade --pre --force-reinstall tox
 
 jobs:
   include:

azure-pipelines.yml

@@ -94,7 +94,7 @@ jobs:
     condition: eq(variables['python.needs_vc'], True)
     displayName: 'Install VC for py27'
 
-  - script: $(python.exe) -m pip install --upgrade pip && $(python.exe) -m pip install tox
+  - script: $(python.exe) -m pip install --upgrade pip && $(python.exe) -m pip install --force-reinstall tox
     displayName: 'Install tox'
 
   - script: |