Use tox release candidates in CI

Fix #2683

.travis.yml

@@ -3,7 +3,8 @@ language: python
 python:
   - '3.5'
 # command to install dependencies
-install: "pip install -U tox"
+install:
+  - pip install --upgrade --pre tox
 # # command to run tests
 env:
   matrix:

appveyor.yml

@@ -36,7 +36,7 @@ install:
 
   - if "%TOXENV%" == "pypy" call scripts\install-pypy.bat
 
-  - C:\Python35\python -m pip install tox
+  - C:\Python35\python -m pip install --upgrade --pre tox
 
 build: false  # Not a C# project, build stuff at the test step instead.