* Add Rails 6 appraisal to start testing against it.
* Fix usage of Module#parent as this is deprecated in Rails 6 in favor
of Module#module_parent.
* Ensure that we're using the correct version of `sqlite3` and `pg` at
all times
* When creating classes within tests, ensure that they are removed
correctly
* Fix detection of has_secure_password in models: Check that
InstanceMethodsOnActivation is specifically defined, as Rails 6 no
longer has such a module.
Co-authored-by: Lee Machin <me@mrl.ee>
