deps: Update regenerator to 0.6

Minor refactoring of `./index.js`, as regenerator's API changed.

Also, updated duo to 0.8.4 so the unit tests can run in node 0.10.

Closes #2.
Merge branch 'master' of github.com:stephenmathieson/duo-regenerator

* 'master' of github.com:stephenmathieson/duo-regenerator:
  Readme: Clarify library purpose