Change dump (with clean flag) and import method for psql

for postgres database, i changed dump with clean flag then import.

normally, we use cap staging db:pull or db:push to sync databases between local and staging for testing, i think this is enough, we don't need to terminate the conection(there is a syntax problem about this, see this issue https://github.com/sgruhier/capistrano-db-tasks/issues/28), dropdb, createdb then import 

what do you think ?