Attempt to fix the failing 'continuous-integration/appveyor/pr' build job

the automated build was failing for the following command:

```
pip install -U pip setuptools wheel

'ERROR: To modify pip, please run the following command:
c:\python35\python.exe -m pip install -U pip setuptools wheel
You are using pip version 10.0.1, however version 18.0 is available.
You should consider upgrading via the 'python -m pip install --upgrade pip' command

```

This is an attempt to fix it: an installation is performed earlier, as
suggested in the error message.

Author: formiaczek

appveyor.yml

@@ -32,10 +32,7 @@ install:
   # Check Python version
   - python --version
 
-  # Upgrade the Python build tools
-  - pip install -U pip setuptools wheel
-
-  # Check pip version
+  # Install Python build tools
   - python -m pip install --upgrade pip setuptools wheel
   - pip --version