Add Python scripts directory to PATH to avoid pip warnings

They're really too annoying.

appveyor.yml

@@ -74,8 +74,9 @@ build_script: c:\projects\wxwidgets\build\tools\appveyor.bat
 before_test:
 - ps: |
     Write-Output "Getting and launching httpbin."
-    C:\Python35\Scripts\pip.exe --disable-pip-version-check install httpbin
-    Start-Job -Name wx_httpbin { C:\Python35\python.exe -m httpbin.core 2>&1 > c:\projects\wxwidgets\httpbin.log }
+    $env:PATH = "C:\Python35;C:\Python35\Scripts;" + $env:PATH
+    pip.exe --disable-pip-version-check install httpbin
+    Start-Job -Name wx_httpbin { python.exe -m httpbin.core 2>&1 > c:\projects\wxwidgets\httpbin.log }
     Start-Sleep -Seconds 5
     if (curl -s http://127.0.0.1:5000/ip > $null) {
         $env:WX_TEST_WEBREQUEST_URL="http://127.0.0.1:5000"