Revert "Use os: Visual Studio 2017 in appveyor.yml"

This reverts commit 87c408b.

CL.exe doesn't seem to be in the path any more when using the
AppVeyor Visual Studio 2017 environment.

msbuild "C:\projects\libsodium\libsodium.vcxproj" /m /verbosity:minimal /logger:"C:\Program Files\AppVeyor\BuildAgent\Appveyor.MSBuildLogger.dll"
Microsoft (R) Build Engine version 15.1.548.43366
Copyright (C) Microsoft Corporation. All rights reserved.
TRACKER : error TRK0005: Failed to locate: "CL.exe". The system cannot find the file specified. [C:\projects\libsodium\libsodium.vcxproj]

appveyor.yml

@@ -1,6 +1,6 @@
 version: 1.0.10.{build}
 
-os: Visual Studio 2017
+os: Visual Studio 2015
 
 environment:
   matrix: