Use Kernel#quietly when checking for pending migrations

The default logger Rails provides as ActiveRecord::Base.logger has a
method called `#silence` that takes no arguments. But ActiveSupport's
[core extension](https://github.com/rails/rails/blob/master/activesupport/lib/active_support/core_ext/kernel/reporting.rb#L82) requires a stream to be passed in, so it would break when
using a regular ::Logger instance.

Switched to using [`Kernel#quietly`](https://github.com/rails/rails/blob/master/activesupport/lib/active_support/core_ext/kernel/reporting.rb#L102), which silences both STDOUT and
STDERR.

I've also added a test for it. I couldn't find any other tests for this piece of code, so I hope I've added it to the right location.