Update CI and move to C++11 due to boost::test requiring it.

.appveyor.yml

@@ -64,40 +64,18 @@ environment:
       B2_CXXSTD: 14,17
       B2_TOOLSET: msvc-14.1
 
-    - FLAVOR: clang-cl
-      APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2017
-      B2_ADDRESS_MODEL: 64
-      B2_CXXSTD: 11,14,17
-      B2_TOOLSET: clang-win
-
-    - FLAVOR: Visual Studio 2015, 2013
-      APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2015
-      B2_TOOLSET: msvc-12.0,msvc-14.0
-
-    - FLAVOR: Visual Studio 2008, 2010, 2012
-      APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2015
-      B2_TOOLSET: msvc-9.0,msvc-10.0,msvc-11.0
-      B2_ADDRESS_MODEL: 32 # No 64bit support
-
     - FLAVOR: cygwin (32-bit)
       APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2017
       ADDPATH: C:\cygwin\bin;
       B2_ADDRESS_MODEL: 32
-      B2_CXXSTD: 03,11,14,1z
+      B2_CXXSTD: 11,14,1z
       B2_TOOLSET: gcc
 
     - FLAVOR: cygwin (64-bit)
       APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2017
       ADDPATH: C:\cygwin64\bin;
       B2_ADDRESS_MODEL: 64
-      B2_CXXSTD: 03,11,14,1z
-      B2_TOOLSET: gcc
-
-    - FLAVOR: mingw64
-      APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2019
-      ADDPATH: C:\mingw-w64\x86_64-8.1.0-posix-seh-rt_v6-rev0\mingw64\bin;
-      B2_ADDRESS_MODEL: 64
-      B2_CXXSTD: 03,11,14,17,2a
+      B2_CXXSTD: 11,14,1z
       B2_TOOLSET: gcc
 
 install: