Fixed issue with container reuse/restart

Use fix from scourou/dokku-pg-plugin@
f4532e12f83a72dc7300f3d4dd282ac99db609c4

Due to the issue Kloadut/dokku-pg-plugin#51
Remove -a args from docker ps command

The 'docker ps -a | ...' command in get_postgresql_id() list all
containers those matched the given app name even they are not running,
in case it has more than one result,  Dokku failed to retrieve the port
of currently running Postgresql container and cannot set a new
DATABASE_URL to the app after rebooting.
Add dokku postgresql:link to the end of pre-deploy

The new port of PG containre is not set into the DATABASE_URL after
rebooting, so the app container is fail to start. Adding a 'dokku
postgresql:link' to the end of pre-deploy file can fixs this.
Merge pull request #59 from superluangs/master

Fix for issue #51 - auto relink db to app container.