Semi-hotfix for Rails configuration bug

At the time Bundler loads NullDB, Rails.root will return nil. We need to
defer calling Rails.root until after Rails loads; would be nice to not
redefine Configuration#project_root, but that does work.

Would not be an issue if people were required to put
`require 'nulldb/rails'` in their projects, perhaps the solution is
moving towards that.

Closes nulldb#16

lib/nulldb/rails.rb

@@ -1,3 +1,4 @@
 require 'nulldb/core'
 
-NullDB.configure {|ndb| ndb.project_root = Rails.root}
+# Need to defer calling Rails.root because when bundler loads, Rails.root is nil
+NullDB.configure {|ndb| def ndb.project_root;Rails.root;end}