Change INSTALL.md for BUILDING.md after release

[jtattermusch]

Once grpc/grpc#15802 gets released and site.data.config.grpc_release_branch is updated.

The occurrences

$ grep -R INSTALL.md *
_posts/2016-4-4-installation.md:C++ | Linux, Mac, Windows | Currently requires [manual build and install](https://github.com/grpc/grpc/blob/{{ site.data.config.grpc_release_branch }}/INSTALL.md)
docs/quickstart/cpp.md:source](https://github.com/grpc/grpc/blob/master/INSTALL.md).
docs/quickstart/cpp.md:instructions for details](https://github.com/grpc/grpc/blob/master/INSTALL.md)).
docs/tutorials/basic/c.md:{{ site.data.config.grpc_release_branch }}/INSTALL.md) first):
docs/tutorials/basic/ruby.md:instructions](https://github.com/grpc/grpc/blob/{{ site.data.config.grpc_release_branch }}/INSTALL.md) first):

[carl-mastrangelo]

Done in #696

[carl-mastrangelo]

Well, partially anyways.