Correct product name and version number for supported version of Visual Studio 2017 (#293)

Author: meastp

README.md

@@ -59,7 +59,7 @@ We are targeting the following compilers:
 
 * gcc 4.8.1
 * clang 3.4
-* MSVC 19
+* Microsoft Visual Studio 2017 15.9 (see [the appveyor documentation](https://www.appveyor.com/docs/windows-images-software/#visual-studio-2017) for the exact version)
 
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