Removed C++03 builds from appveyor.

Author: redboltz

appveyor.yml

@@ -6,16 +6,6 @@ branches:
 
 environment:
   matrix:
-    - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2013
-      cpp11: -DMSGPACK_CXX11=OFF
-      msvc: '"Visual Studio 12 2013"'
-      boost_prefix: C:\Libraries\boost_1_58_0
-      boost_subdir: lib32-msvc-12.0
-    - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2015
-      cpp11: -DMSGPACK_CXX11=OFF
-      msvc: '"Visual Studio 14 2015"'
-      boost_prefix: C:\Libraries\boost_1_69_0
-      boost_subdir: lib32-msvc-14.0
     - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2015
       cpp11: -DMSGPACK_CXX11=ON
       msvc: '"Visual Studio 14 2015"'