Remove the side-effects of validates_presence_of.

The validation added to Speedometer class will result in failures of other tests such as [this](https://github.com/rails/rails/blob/master/activerecord/test/cases/associations/has_many_associations_test.rb#L1881) and [this](https://github.com/rails/rails/blob/master/activerecord/test/cases/calculations_test.rb#L259), with the error `Validation failed: Dashboard can't be blank`.

cc @senny