Fix ImageDebugDirectory.MinorVersion in portable pdb writer
Fix ImageDebugDirectory.MinorVersion in portable pdb writer

The minor version is wrong, it should be 20557 (0x504d) according to the spec: https://github.com/dotnet/corefx/blob/fd15ab1de03a4431e0ead9ff10e1bf53e2ae4bb7/src/System.Reflection.Metadata/specs/PE-COFF.md#codeview-debug-directory-entry-type-2

Fixes https://github.com/jbevain/cecil/issues/309
jbevain/cecil,sailro/cecil,fnajera-rac-de/cecil,SiliconStudio/Mono.Cecil,mono/cecil