Update dependencies.
Update test helper.
Update slug tests.
Update Ohm::Timestamps tests.
Fix Ohm::Timestamps tests.
Update Ohm::Scope tests.
Update Ohm::Callbacks tests.
Fix Ohm::Callbacks tests.
Update Ohm::DataTypes tests.
Casts symbol only if exists.
Remove unneeded check. Ohm 2 doesn't save values for attributes set to
false.

Related commit
https://github.com/soveran/ohm/commit/717080fdb9ab9f71ee19cf84650dd6e5f6cc1789.
Use stabby lambda syntax.
Remove unused files.
Use makefile instead of Rakefile.
Fix ohm version on gemspec.
Update Ohm::SoftDelete tests.
Fix Ohm::SoftDelete tests.
Update Ohm::SoftDelete documentation.
Remove duplicated tests about soft delete.
Update Ohm::Locking tests.