Unlink the Java GCM from Homebrew if installed

[mjcheetham]

If the Java GCM is installed via Homebrew, attempt to unlink it before creating the GCM Core symlink in /usr/local/bin.

Also include the -F option when creating the symlink for good measure to force overwriting whatever file might already be there. Technically we only need to do this step and not the brew unlink but we should try and let Homebrew remove the links it placed rather than just walking over them.

Fixes #44

[kewillford]

Looks good to me.

[jamill]

Looks good! Thanks. Just a couple of thoughts for your consideration.

Are there any details that we would want to include in the README?

I see we unlink java GCM - I suspect that most users would could just remove Java GCM. It is probably not the place of your installer to do this, but maybe a note in the Readme / installation instructions could indicate that this replaces the Java GCM and how they could remove it if they wanted to?

[mjcheetham]

Looks good! Thanks. Just a couple of thoughts for your consideration.

Are there any details that we would want to include in the README?

I see we unlink java GCM - I suspect that most users would could just remove Java GCM. It is probably not the place of your installer to do this, but maybe a note in the Readme / installation instructions could indicate that this replaces the Java GCM and how they could remove it if they wanted to?

Updated the "Download and Install" instructions on the README.md to include a note that the Java GCM will be unlinked if currently installed.