Fix puma_bind unix socket path

I don't know what was the decision behind setting all paths with `File.join` instead of an interpolated string, and if it is worth rolling back (so I didn't), but `File.join` seems to mess up the unix socket URL:

    > File.join('unix://', '/home/deploy/app', 'tmp', 'sockets', 'puma.socket') # => "unix:/home/deploy/app/tmp/sockets/puma.socket"
    > File.join('unix://home/deploy/app', 'tmp', 'sockets', 'puma.socket') # => "unix://home/deploy/app/tmp/sockets/puma.socket"