Sort appveyor entries. Add CMAKE_SYSTEM_VERSION=8.1 in the hope it fixes the build.

appveyor.yml

diff --git a/appveyor.yml b/appveyor.yml
index 2061365..c56270b 100644
--- a/appveyor.yml
+++ b/appveyor.yml
@@ -5,27 +5,27 @@
     - GENERATOR: "Visual Studio 15 2017 Win64"
       TRIPLET: x64-windows
       CMAKE_OPTS: "-DBUILD_SHARED_LIBS=off"
+    - GENERATOR: "Visual Studio 15 2017 Win64"
+      TRIPLET: x64-uwp
+      CMAKE_OPTS: "-DCMAKE_SYSTEM_NAME=WindowsStore -DCMAKE_SYSTEM_VERSION=8.1"
     - GENERATOR: "Visual Studio 15 2017"
       TRIPLET: x86-windows
       CMAKE_OPTS: "-DBUILD_SHARED_LIBS=off"
+    - GENERATOR: "Visual Studio 15 2017"
+      TRIPLET: x86-uwp
+      CMAKE_OPTS: "-DCMAKE_SYSTEM_NAME=WindowsStore -DCMAKE_SYSTEM_VERSION=8.1"
     - GENERATOR: "Visual Studio 15 2017 ARM"
       TRIPLET: arm-windows
       CMAKE_OPTS: "-DENABLE_OPENSSL=off"
-    - GENERATOR: "Visual Studio 15 2017"
-      TRIPLET: x86-uwp
-      CMAKE_OPTS: "-DCMAKE_SYSTEM_NAME=WindowsStore"
-    - GENERATOR: "Visual Studio 15 2017 Win64"
-      TRIPLET: x64-uwp
-      CMAKE_OPTS: "-DCMAKE_SYSTEM_NAME=WindowsStore"
     - GENERATOR: "Visual Studio 15 2017 ARM"
       TRIPLET: arm-uwp
-      CMAKE_OPTS: "-DCMAKE_SYSTEM_NAME=WindowsStore -DENABLE_OPENSSL=off"
+      CMAKE_OPTS: "-DCMAKE_SYSTEM_NAME=WindowsStore -DCMAKE_SYSTEM_VERSION=8.1 -DENABLE_OPENSSL=off"
     - GENERATOR: "Visual Studio 15 2017"
       TRIPLET: arm64-windows
       CMAKE_OPTS: "-AARM64 -DENABLE_OPENSSL=off"
     - GENERATOR: "Visual Studio 15 2017"
       TRIPLET: arm64-uwp
-      CMAKE_OPTS: "-AARM64 -DCMAKE_SYSTEM_NAME=WindowsStore -DENABLE_OPENSSL=off"
+      CMAKE_OPTS: "-AARM64 -DCMAKE_SYSTEM_NAME=WindowsStore -DCMAKE_SYSTEM_VERSION=8.1 -DENABLE_OPENSSL=off"
 before_build:
   cmd: >-
     git clone https://github.com/Microsoft/vcpkg