what about removing the silent-postgres gem?

When running the test, there is too much waring about the silence method of Logger which has been removed in rails 4.
```
DEPRECATION WARNING: silence is deprecated and will be removed from Rails 4.0. (called from pk_and_sequence_for_with_silencer at (eval):2)
```
So I think we can remove the silent-postgres gem if it is not so useful.
https://github.com/dolzenko/silent-postgres/issues/19