[Appveyor] Invalidate Conan cache on change in conanfile.py.

.appveyor.yml

@@ -17,7 +17,7 @@ configuration:
   - Release
 
 cache:
-  - C:\projects\deps\conan
+  - C:\projects\deps\conan -> conanfile.py
 
 install:
   # All external dependencies are installed in C:\projects\deps

conanfile.py

+## Appveyor cache version: 1
 from conans import ConanFile, CMake
 
 class OpenGeoSysConan(ConanFile):