Fix jdbc:postgresql url format to use 'user' instead 'username'.

When using a jdbc:postgresql url valid according to the current ParserURL (e.g. `jdbc:postgresql://localhost:5432/mydb?username=foo&password=bar`) the jdbc driver (used by the evolutions plugin) cannot connect to the db as it's missing the user connection parameter.

According to the [postgresql jdbc driver documentation](http://jdbc.postgresql.org/documentation/91/connect.html#connection-parameters) the correct parameter is 'user' instead of the previously used 'username' (the url becomes e.g. `jdbc:postgresql://localhost:5432/mydb?user=foo&password=bar`) - this shouldn't have worked for anyone.