during clone-repo task for dryad-repo, accept github's ssh hostkey

During a vagrant provision for a dryad test run today, I encountered this failure to clone the dryad-repo:

```
TASK: [dryad_app | Clone Dryad repo] ******************************************
failed: [vagrant-dryad] => {"failed": true}
msg: github.com has an unknown hostkey. Set accept_hostkey to True or manually add the hostkey prior to running the git module

FATAL: all hosts have already failed -- aborting

PLAY RECAP ********************************************************************
           to retry, use: --limit @/Users/rnathanday/setup.retry

vagrant-dryad              : ok=16   changed=2    unreachable=0    failed=1

Ansible failed to complete successfully. Any error output should be
visible above. Please fix these errors and try again.
```
It was not fatal, but the provisioned VM had no dryad-repo.