In looking into #563, I realized there are gaps in test coverage. For example, `bin/annotate` doesn't have any tests. 

Tidying the spec directory to make it easier to add tests in future work.

- Use Rake::Task#enhance insteaad of defining same tasks again
- Remove hooking db:migrate:change task which doesn't exist
- Fix hooking db:migrate:reset task so that the annotation runs after
  all migration tasks (#548)