Change the appveyor build to use the minimal required MSVC version

[Naios]

The Visual Studio version used by the Appveyor CI should reflect the minimal required MSVC version
2015 instead of the latest one.
This could help to prevent issues where the code was developed in a later MSVC version but fails to build on the minimal required one.

[hkaiser]

@Naios: this can be easily done by changing this line: https://github.com/STEllAR-GROUP/hpx/blob/master/appveyor.yml#L9 to read Visual Studio 2015. We could even have both versions in there (see: https://www.appveyor.com/docs/build-environment/#using-multiple-images-for-the-same-build).

[Naios]

@hkaiser Thanks for the quick reply, I'll try that out.