appveyor: try building UWP ports for Windows 10 (for bcrypt) instead of 8.1

appveyor.yml

diff --git a/appveyor.yml b/appveyor.yml
index c56270b..a9745c7 100644
--- a/appveyor.yml
+++ b/appveyor.yml
@@ -7,25 +7,25 @@
       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"
+      CMAKE_OPTS: "-DCMAKE_SYSTEM_NAME=WindowsStore -DCMAKE_SYSTEM_VERSION=10"
     - 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"
+      CMAKE_OPTS: "-DCMAKE_SYSTEM_NAME=WindowsStore -DCMAKE_SYSTEM_VERSION=10"
     - GENERATOR: "Visual Studio 15 2017 ARM"
       TRIPLET: arm-windows
       CMAKE_OPTS: "-DENABLE_OPENSSL=off"
     - GENERATOR: "Visual Studio 15 2017 ARM"
       TRIPLET: arm-uwp
-      CMAKE_OPTS: "-DCMAKE_SYSTEM_NAME=WindowsStore -DCMAKE_SYSTEM_VERSION=8.1 -DENABLE_OPENSSL=off"
+      CMAKE_OPTS: "-DCMAKE_SYSTEM_NAME=WindowsStore -DCMAKE_SYSTEM_VERSION=10 -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 -DCMAKE_SYSTEM_VERSION=8.1 -DENABLE_OPENSSL=off"
+      CMAKE_OPTS: "-AARM64 -DCMAKE_SYSTEM_NAME=WindowsStore -DCMAKE_SYSTEM_VERSION=10 -DENABLE_OPENSSL=off"
 before_build:
   cmd: >-
     git clone https://github.com/Microsoft/vcpkg