windows: Make appveyor.yml self-contained.

Add configuration which otherwise has to be set via the UI so the file is
more self-contained, and remove configuration which is not needed because
it's the same as the default.  The major change here is that for a while
now Appveyor has been using Visual Studio 2015 by default while we still
want to support 2013.

ports/windows/.appveyor.yml

@@ -1,3 +1,7 @@
+image: Visual Studio 2013
+clone_depth: 1
+skip_tags: true
+
 environment:
   # Python version used
   MICROPY_CPYTHON3: c:/python34/python.exe
@@ -71,10 +75,3 @@ after_test:
     if ($LASTEXITCODE -ne 0) {
       throw "$env:MSYSTEM mpy-cross tests exited with code $LASTEXITCODE"
     }
-
-skip_tags: true
-
-deploy: off
-
-nuget:
-  disable_publish_on_pr: true