Use old pip upgrade command only for Python 2.6 in appveyor.yml

appveyor.yml

@@ -16,6 +16,8 @@ environment:
     # /E:ON and /V:ON options are not enabled in the batch script intepreter
     # See: http://stackoverflow.com/a/13751649/163740
     CMD_IN_ENV: "cmd /E:ON /V:ON /C .\\appveyor\\run_with_env.cmd"
+    # This command works for most versions of Python in AppVeyor except of Python 2.6
+    PIP_UPGRADE_CMD: "python -m pip install --disable-pip-version-check --user --upgrade pip"
     #PANDOC_URL: "https://github.com/jgm/pandoc/releases/download/1.17.1/pandoc-1.17.1-1-windows.msi"
     #PANDOC_MSI: "C:\\pandoc.msi"
 
@@ -27,6 +29,8 @@ environment:
     - PYTHON: "C:\\Python26"
       PYTHON_VERSION: "2.6.x" # currently 2.6.6
       PYTHON_ARCH: "32"
+      # For Python 2.6 we are using old version of pip upgrade command
+      PIP_UPGRADE_CMD: "pip install --disable-pip-version-check --user --upgrade pip"
 
     - PYTHON: "C:\\Python26-x64"
       PYTHON_VERSION: "2.6.x" # currently 2.6.6
@@ -83,7 +87,7 @@ install:
 
   # Upgrade to the latest version of pip to avoid it displaying warnings
   # about it being out of date.
-  - "python -m pip install --disable-pip-version-check --user --upgrade pip"
+  - "%PIP_UPGRADE_CMD%"
 
   # Install requirements
   - "%CMD_IN_ENV% pip install -r requirements.txt"