[RFC] Add coverity build type.

Second attempt, this time with a more reviewable PR.

After merging this, @aktau would need to re-encrypt the coverity token for Travis:

```bash
cd bot-ci
travis encrypt 'COVERITY_SCAN_TOKEN=<token>' -r neovim/bot-ci --add
```

We already have a `secret` entry containing `GH_TOKEN`, but this is not a problem:

> You may add multiple entries to your .travis.yml with key "secure." They will all be available to your program.