Changed documentation of converting a MySQL-Database with Gitlab to a Postgresdatatabase. The instructions were missleading. See -> https://gitlab.com/gitlab-org/gitlab-ce/issues/2904

Closes #2176 (gitlab.com)

Introduced in
https://github.com/gitlabhq/mysql-postgresql-converter/commit/b2d1599706f532e1bac836243f25e005c816ef14

[ci skip]

Running converter with Python 2.6.x installed gives the following error:
AttributeError: 'module' object has no attribute 'check_output'

After installing Python 2.7.x everything works fine.

This ensures that all tables created during the import belong to `git`.
If you import as a different user, such as the `postgres` superuser, you
may encounter issues where the GitLab database user cannot access tables
in gitlabhq_production, _even if_ `git` is the owner of
gitlabhq_production at the time of import.