Remove support for Visual C++ 2015.

.appveyor.yml

@@ -1,27 +1,14 @@
-image:
-  - Visual Studio 2015
-  - Visual Studio 2017
+image: Visual Studio 2017
 
 platform: x64
 configuration: Release
 
 version: ci.{build}
 
+before_build:
+  - COPY .ci\VS2017.props .\Build.props
+
 build:
   parallel: false
   project: QuantLib.sln
   verbosity: normal
-
-for:
--
-  matrix:
-    only:
-      - image: Visual Studio 2015
-  before_build:
-    - COPY .ci\VS2015.props .\Build.props
--
-  matrix:
-    only:
-      - image: Visual Studio 2017
-  before_build:
-    - COPY .ci\VS2017.props .\Build.props

ql/config.msvc.hpp

@@ -25,8 +25,8 @@
 #include <ql/userconfig.hpp>
 
 // first things first
-#if (_MSC_VER < 1900)
-#  error "versions of Visual C++ prior to VC++ 2015 are no longer supported"
+#if (_MSC_VER < 1910)
+#  error "versions of Visual C++ prior to VC++ 2017 are no longer supported"
 #endif
 
 /*******************************************

ql/errors.cpp

@@ -22,7 +22,7 @@
 
 namespace {
 
-    #if defined(_MSC_VER) || defined(__BORLANDC__)
+    #if defined(_MSC_VER)
     // allow Visual Studio integration
     std::string format(
                        #ifdef QL_ERROR_LINES

ql/handle.hpp

@@ -127,11 +127,7 @@ namespace QuantLib {
         /*! \deprecated Use one of the constructors taking shared_ptr.
                         Deprecated in version 1.35.
         */
-#if defined(_MSC_VER) && (_MSC_VER < 1916)
-        QL_DEPRECATED
-#else
         [[deprecated("Use one of the constructors taking shared_ptr.")]]
-#endif
         explicit RelinkableHandle(
                        T* p,
                        bool registerAsObserver = true);