Use --force-reinstall when installing tox

As described in:

tox-dev/tox#791

Author: Bruno Oliveira

.travis.yml

@@ -59,7 +59,7 @@ jobs:
 
 install:
   - pip install -U pip
-  - pip install -U setuptools tox
+  - pip install -U --force-reinstall setuptools tox
 
 script:
   - tox

appveyor.yml

@@ -27,7 +27,7 @@ install:
   - pypy --version
 
   - C:\Python35\python -m pip install -U pip
-  - C:\Python35\python -m pip install -U setuptools tox
+  - C:\Python35\python -m pip install -U --force-reinstall setuptools tox
 
 build: false  # Not a C# project, build stuff at the test step instead.