System.Numerics.Vectors.sln: use correct configuration for Debug

Before, the Release target got incorrectly assigned to the Debug configuration
for the library and test projects. This results in the library being compiled
into the Release dir when the solution is compiled on the command line
via msbuild.exe, while it should be compiled into the Debug dir.
Merge pull request #285 from akoeplinger/patch-1

System.Numerics.Vectors.sln: use correct configuration for Debug