Hotfix release: it was not possible to apply database migrations in version 0.4.2.

This is a bugfix release.

## Fixes

* SQLite failed with the error "database table locked" when the database is shared by several Gateway instances. (#273)
* An error occured during the import if a new local server was created while another instance already had one with the same name. (#272)
* removal of the '/' character at the beginning of the paths returned in the answer to the SFTP command `ls`.
* A unicity constraint on the history database tables caused some errors. (#265)
* The REST API returned wrong authorizations in the rules. (#266)
* An SFTP server  could start without an SSH key, which would cause connection errors. The error is now logged when the server starts. (#267)