update to Rails 6

[chvp]

Three minor issues remain:

• We need to use an unofficial fork of Foreman to make sure we can update thor
• The coverage result is wrong if we want to use the new rails utility to parallelize tests (SimpleCov showing incorrect coverage result when parallelize is enabled in Rails 6.0.0 beta3 simplecov-ruby/simplecov#718)
• CI can't take advantage of the parallelized tests: the mariadb docker container closes all but one connection prematurely (at least that's what I think is happening).

[bmesuere]

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