Delay requiring solr_document until we're actually loading an instance of Configuration

I was trying to update Spotlight to use blacklight master and got a bunch of weird initialization errors, e.g.:

```
/Volumes/TempStorage/Projects/spotlight/spec/internal/config/initializers/friendly_id.rb:12:in `<top (required)>': uninitialized constant FriendlyId (NameError)
	from /Users/cabeer/.rubies/ruby-2.1.1/lib/ruby/gems/2.1.0/gems/railties-4.0.4/lib/rails/engine.rb:609:in `block (2 levels) in <class:Engine>'
	from /Users/cabeer/.rubies/ruby-2.1.1/lib/ruby/gems/2.1.0/gems/railties-4.0.4/lib/rails/engine.rb:608:in `each'
	from /Users/cabeer/.rubies/ruby-2.1.1/lib/ruby/gems/2.1.0/gems/railties-4.0.4/lib/rails/engine.rb:608:in `block in <class:Engine>'
```

I don't fully understand how the error I was seeing was related to this change, but it must be a runtime load error of some sort.