Better appveyor file setup.

Author: eldiener

appveyor.yml

@@ -11,6 +11,13 @@ branches:
     - master
     - develop
 
+environment:
+  matrix:
+    - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2015
+      TOOLSET: msvc-9.0,msvc-10.0,msvc-11.0,msvc-12.0,msvc-14.0
+    - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2017
+      TOOLSET: msvc-14.1
+
 install:
   - cd ..
   - git clone -b %APPVEYOR_REPO_BRANCH% https://github.com/boostorg/boost.git boost-root
@@ -26,4 +33,4 @@ install:
 build: off
 
 test_script:
-  - b2 libs/lambda/test toolset=msvc-9.0,msvc-10.0,msvc-11.0,msvc-12.0,msvc-14.0,msvc-14.1
+  - b2 libs/lambda/test toolset=%TOOLSET%