Adding method check_pending to module RSpec

This is method is to replace the statement:
```ruby
# Checks for pending migrations before tests are run.
# If you are not using ActiveRecord, you can remove this line.
ActiveRecord::Migration.check_pending! if defined?(ActiveRecord::Migration)
```
that goes into **spec_helper.rb**.

It should change to:
```ruby
# Checks for pending migrations before tests are run.
# If you are not using ActiveRecord, you can remove this line.
RSpec::check_pending if defined?(ActiveRecord::Migration)
```

It shows a prettier message than rising a exception.