enable retries for WB => Dropbox uploads
enable retries for WB => Dropbox uploads

 * Dropbox responds to too many simultaneous upload requests with a
   429 error.  WB has the ability to automatically retry failed
   requests with backoff, but 429 is not in the set of status codes
   that will automatically retry.  Add it.

   [SVCS-401]

TomBaxter/waterbutler,RCOSDP/waterbutler,felliott/waterbutler,rdhyee/waterbutler,CenterForOpenScience/waterbutler