Extend key validity till 2016-02-24.

* Extend key validity till 2016-02-24.
* Install keyring to /usr/share/postgresql-common/pgdg/ and import it using
  apt-key --keyring /etc/apt/trusted.gpg.d/apt.postgresql.org.gpg.
* Remove our key from trusted.gpg if present so there are not two copies.