Redic::Pool#commit fails if the queue is empty

The `commit` method fails with a NoMethodError on `nil` if the queue is empty. This is because `Thread.current[@id]` never gets initialized. Here's a quick fix:

```ruby
class Redic::Pool
  # https://github.com/djanowski/redic-pool/issues/7
  alias_method :unsafe_commit, :commit

  def commit
    Thread.current[@id] || (Thread.current[@id] = [])

    unsafe_commit
  end
end
```