Remove Visual Studio 2015 support; require Visual Studio 2017.

VS 2015 is just too buggy. Working around the bugs is wasting developer time.

This change does not actually remove any of the hacks yet, so that we can undo this decision if there are significant complaints after the next release.

appveyor.yml

@@ -7,8 +7,8 @@
 #     support for.
 #   - Use CMake to ...
 #       build Cap'n Proto with MinGW.
-#       build Cap'n Proto with VS2015 and VS2017.
-#       build Cap'n Proto samples with VS2015 and VS2017.
+#       build Cap'n Proto with VS2017.
+#       build Cap'n Proto samples with VS2017.
 
 version: "{build}"
 
@@ -42,10 +42,6 @@ environment:
       # TODO(someday): Right now /maxcpucount occasionally expresses a filesystem-related race:
       #   capnp-capnpc++ complains that it can't create test.capnp.h.
 
-    - CMAKE_GENERATOR: Visual Studio 14 2015
-      BUILD_NAME: vs2015
-      EXTRA_BUILD_FLAGS: # /maxcpucount
-
     - CMAKE_GENERATOR: MinGW Makefiles
       BUILD_NAME: mingw
       EXTRA_BUILD_FLAGS: -j2

c++/src/kj/common.h

@@ -60,8 +60,8 @@
     #endif
   #endif
 #elif defined(_MSC_VER)
-  #if _MSC_VER < 1900
-    #error "You need Visual Studio 2015 or better to compile this code."
+  #if _MSC_VER < 1910
+    #error "You need Visual Studio 2017 or better to compile this code."
   #endif
 #else
   #warning "I don't recognize your compiler.  As of this writing, Clang and GCC are the only "\