LTI update task

This task will update the LTI key and secret for the current envirnoment (so e.g., if you want to update the production environment type:

```bash
RAILS_ENV=production thor keys:lti_new
```

It also adds a task to list the currently configured api key and lti key and secret:

```bash
RAILS_ENV=production thor keys:list
```

A soon to be issued PR will actually go in and update the running canvas instance for the 3 'LTI apps' (Leaderboard/Engagement Index, Gallery, and PointsConfiguration).