Dropped Visual Studio 2015 support

Author: yhirose

README.md

@@ -966,7 +966,7 @@ Include `httplib.h` before `Windows.h` or include `Windows.h` by defining `WIN32
 > cpp-httplib officially supports only the latest Visual Studio. It might work with former versions of Visual Studio, but I can no longer verify it. Pull requests are always welcome for the older versions of Visual Studio unless they break the C++11 conformance.
 
 > [!NOTE]
-> Windows 8 or lower, Visual Studio 2013 or lower, and Cygwin and MSYS2 including MinGW are neither supported nor tested.
+> Windows 8 or lower, Visual Studio 2015 or lower, and Cygwin and MSYS2 including MinGW are neither supported nor tested.
 
 License
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