Undefined symbols for architecture arm64: "_OBJC_CLASS_$_P31MonoBridge"

While building my xCode project from Unity, I am getting this kind of error as result when I try to build a project. I have added only one plugin - GameKit and that is very old that I know but any how I want to use this plugin because I don’t have latest version of it.
My xCode version is 7.0.1 and Unity version is 4.6.3.

Here I am attaching screenshots for give you more idea about error I am getting.

Architectures Section

Debug Section

This may only be solved by acquiring a newer version of the Prime31 plugin for GameCenter, this error looks to be suggesting that the version you are currently using simply is not compatible with 64 bit builds. This is only solvable by not doing a 64bit build (not an option if you want to distribute on the apple store) or update the plugin to support that architecture.