Add msvc-12.0 to Appveyor

appveyor.yml

@@ -11,6 +11,10 @@ branches:
     - master
     - develop
 
+env:
+  - TOOLSET=msvc-12.0
+  - TOOLSET=msvc-14.0
+
 install:
   - cd ..
   - git clone -b %APPVEYOR_REPO_BRANCH% --depth 1 https://github.com/boostorg/boost.git boost-root
@@ -27,4 +31,4 @@ install:
 build: off
 
 test_script:
-  - b2 libs/mp11/test toolset=msvc-14.0
+  - b2 libs/mp11/test toolset=$TOOLSET