Make Protobuf compatible with C# 6

[ghost]

Fix internal C# compilation issue.

[ObsidianMinor]

You can also set the language version to C# 6 in each project (<LangVersion>6</LangVersion>) to prevent future instances of incompatibility.

[ghost]

You can also set the language version to C# 6 in each project (<LangVersion>6</LangVersion>) to prevent future instances of incompatibility.

Thanks for the suggestion! Done.

[anandolee]

Thanks for the fix!