Update appveyor.yml

appveyor.yml

@@ -68,13 +68,13 @@ build_script:
 before_test:
 - cmd: >-
 
-    C:\Python27\python.exe -m pip install --find-links=dist\ opencv-python
+    C:\Python27\python.exe -m pip install --ignore-installed --find-links=dist\ opencv-python
 
-    C:\Python27-x64\python.exe -m pip install --find-links=dist\ opencv-python
+    C:\Python27-x64\python.exe -m pip install --ignore-installed --find-links=dist\ opencv-python
 
-    C:\Python35\python.exe -m pip install --find-links=dist\ opencv-python
+    C:\Python35\python.exe -m pip install --ignore-installed --find-links=dist\ opencv-python
 
-    C:\Python35-x64\python.exe -m pip install --find-links=dist\ opencv-python
+    C:\Python35-x64\python.exe -m pip install --ignore-installed --find-links=dist\ opencv-python
 
 test_script:
 - cmd: >-
@@ -89,4 +89,4 @@ test_script:
 
 artifacts:
 - path: dist\*.whl
-  name: wheels
\ No newline at end of file
+  name: wheels