Bump AppVeyor to use Visual Studio 2015

This is a work around for sass/libsass#1719 which removed a header
that broke compilation on VS 2013.

appveyor.yml

@@ -1,4 +1,6 @@
 -
+  os: Visual Studio 2015
+
   branches:
     only:
       - release
@@ -34,7 +36,7 @@
     - node --version
     - npm --version
     - git submodule update --init --recursive
-    - npm install --msvs_version=2013
+    - npm install --msvs_version=2015
 
   test_script: npm test
 
@@ -55,6 +57,8 @@
         appveyor_repo_tag: true       # deploy on tag push only
 
 -
+  os: Visual Studio 2015
+
   configuration: testing
 
   platform:
@@ -83,6 +87,6 @@
     - node --version
     - npm --version
     - git submodule update --init --recursive
-    - npm install --msvs_version=2013
+    - npm install --msvs_version=2015
 
   test_script: npm test