Bump version to 15.9
Bump version to 15.9

chipitsine/msbuild,jeffkl/msbuild,jeffkl/msbuild,jeffkl/msbuild,chipitsine/msbuild,sean-gilliam/msbuild,Microsoft/msbuild,chipitsine/msbuild,cdmihai/msbuild,mono/msbuild,cdmihai/msbuild,Microsoft/msbuild,AndyGerlicher/msbuild,rainersigwald/msbuild,cdmihai/msbuild,sean-gilliam/msbuild,sean-gilliam/msbuild,sean-gilliam/msbuild,rainersigwald/msbuild,mono/msbuild,Microsoft/msbuild,chipitsine/msbuild,cdmihai/msbuild,jeffkl/msbuild,Microsoft/msbuild,AndyGerlicher/msbuild,rainersigwald/msbuild,mono/msbuild,AndyGerlicher/msbuild,rainersigwald/msbuild,AndyGerlicher/msbuild,mono/msbuild