Changed dependency on 'mongoose-attachments'

- moved dependency on 'mongoose-attachments' to 'devDependencies'
  - when 'mongoose-attachments' is a dependency (not devDependency), the
    module reads its copy vs the host module's, preventing the use of
    two strategies (localfs & knox)
- removed reference to a branch of a fork of the 'mongoose-attachments' module
- added reference to the latest 'mongoose-attachments' module