upgrade database version to beta2

There are no more model changes going into beta2, those planned are postponed to beta3.

@joshmoore, @pwalczysko: One may wish to exclude this PR until we are ready to purge gretzky's database.

As part of testing, one may wish to verify that the system works okay after an upgrade from 4.4 via `sql/psql/OMERO5.0DEV__6/OMERO4.4__0.sql` then `sql/psql/OMERO5.0__0/OMERO5.0DEV__6.sql`. Also it should work okay with a database created from `OMERO5.0__0.sql` generated by `bin/omero db script`. Update: see https://github.com/openmicroscopy/openmicroscopy/pull/1842#issuecomment-29715108.

--no-rebase