appveyor: Update to MSVC 2017.

That's what we (and I suppose most people out there) are using now.

Reviewed-by: Roland Scheidegger <sroland@vmware.com>

appveyor.yml

@@ -35,9 +35,9 @@ clone_depth: 100
 
 cache:
 - win_flex_bison-2.5.9.zip
-- llvm-5.0.1-msvc2015-mtd.7z
+- llvm-5.0.1-msvc2017-mtd.7z
 
-os: Visual Studio 2015
+os: Visual Studio 2017
 
 init:
 # Appveyor defaults core.autocrlf to input instead of the default (true), but
@@ -46,7 +46,7 @@ init:
 
 environment:
   WINFLEXBISON_ARCHIVE: win_flex_bison-2.5.9.zip
-  LLVM_ARCHIVE: llvm-5.0.1-msvc2015-mtd.7z
+  LLVM_ARCHIVE: llvm-5.0.1-msvc2017-mtd.7z
 
 install:
 # Check git config
@@ -61,7 +61,7 @@ install:
 # Install python wheels, necessary to install SCons via pip
 - python -m pip install wheel
 # Install SCons
-- python -m pip install scons==2.5.1
+- python -m pip install scons==3.0.1
 - scons --version
 # Install flex/bison
 - if not exist "%WINFLEXBISON_ARCHIVE%" appveyor DownloadFile "https://downloads.sourceforge.net/project/winflexbison/old_versions/%WINFLEXBISON_ARCHIVE%"
@@ -76,10 +76,10 @@ install:
 - set LLVM=%CD%\llvm
 
 build_script:
-- scons -j%NUMBER_OF_PROCESSORS% MSVC_VERSION=14.0 llvm=1
+- scons -j%NUMBER_OF_PROCESSORS% MSVC_VERSION=14.1 llvm=1
 
 after_build:
-- scons -j%NUMBER_OF_PROCESSORS% MSVC_VERSION=14.0 llvm=1 check
+- scons -j%NUMBER_OF_PROCESSORS% MSVC_VERSION=14.1 llvm=1 check
 
 
 # It's possible to setup notification here, as described in