appveyor.yml: Revert back to Visual Studio 2015 image

Apparently AppVeyor dropped support for some compilation environments in
the 2017 image, so fall back to the 2015 image and forego compilation
with the newer tools.

Signed-off-by: Chris Dickens <christopher.a.dickens@gmail.com>

appveyor.yml

diff --git a/appveyor.yml b/appveyor.yml
index 676a543..ae4704c 100644
--- a/appveyor.yml
+++ b/appveyor.yml
@@ -1,5 +1,5 @@
 version: 1.0.{build}
-image: Visual Studio 2017
+image: Visual Studio 2015
 configuration:
 - Debug
 - Release
@@ -7,13 +7,12 @@
 - x64
 - Win32
 build:
-  project: msvc\libusb_2017.sln
+  project: msvc\libusb_2015.sln
   parallel: true
   verbosity: detailed
 environment:
   matrix:
-  - libusb_2017: msvc\libusb_2017.sln
-    libusb_2015: msvc\libusb_2015.sln
+  - libusb_2015: msvc\libusb_2015.sln
     libusb_2013: msvc\libusb_2013.sln
 install:
 - cmd: >-
@@ -26,8 +25,6 @@
     xcopy /S C:\projects\libusb C:\msys64\home\appveyor\
 build_script:
 - cmd: >-
-    msbuild %libusb_2017% /logger:"C:\Program Files\AppVeyor\BuildAgent\Appveyor.MSBuildLogger.dll"
-
     msbuild %libusb_2015% /logger:"C:\Program Files\AppVeyor\BuildAgent\Appveyor.MSBuildLogger.dll"
 
     msbuild %libusb_2013% /logger:"C:\Program Files\AppVeyor\BuildAgent\Appveyor.MSBuildLogger.dll"

libusb/version_nano.h

diff --git a/libusb/version_nano.h b/libusb/version_nano.h
index a8068c6..d7c69b6 100644
--- a/libusb/version_nano.h
+++ b/libusb/version_nano.h
@@ -1 +1 @@
-#define LIBUSB_NANO 11438
+#define LIBUSB_NANO 11439