Fixed issues with sails-postgresql in DatabaseTasks

* DatabaseTasks.drop/create didn't return err/adapter to the
 calling function.
* Fixed issues with the specs where a loaded sails prevented
 from another sails version to be loaded in the same process
 Now specs determine which sails version to test against
 the SAILS_VERSION env variable
* Issue with Waterline - can't close open connections, so
 changed the specs to use dropSchema instead of dropDB, so
 PostgresSQL won't cause issues, as it prevents the command
 from running while there are open connections
* Added PostgreSQL to TravisCI, using the SAILS_ADAPTER_NAME
 env variable to determine which adapter to use for sails
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