Create incidents Atom feed

Offer Atom feeds on `/incidents.atom`, as suggested in here: https://github.com/intercom/banjaxed/issues/38
* `/incidents.rss` gets redirected to the Atom one.
* The `updated` field of the field is set to the date of the date an incident was updated last.
* The unittest checks that when the atom format is requested, that date is set.

```bash
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Finished in 1.05 seconds (files took 1.72 seconds to load)
68 examples, 0 failures

Randomized with seed 28211

Coverage report generated for RSpec to /Users/mmolina/work/banjaxed/coverage. 106 / 106 LOC (100.0%) covered.
```