[RFC] Add coverity build type.

Second attempt, this time with a more reviewable PR.

After merging this, @aktau (I think, based on `COVERITY_SCAN_NOTIFICATION_EMAIL`) 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.

I don't have any way to really test this PR, unfortunately, but at least there are no syntax errors:

> $ ./ci/${CI_TARGET}.sh
Cloning into '/home/travis/build/fwalch/bot-ci/build/neovim'...
remote: Counting objects: 1948, done.
remote: Compressing objects: 100% (1771/1771), done.
remote: Total 1948 (delta 156), reused 1645 (delta 146)
Receiving objects: 100% (1948/1948), 6.86 MiB | 13.46 MiB/s, done.
Resolving deltas: 100% (156/156), done.
Checking connectivity... done.
Note: COVERITY_SCAN_PROJECT_NAME and COVERITY_SCAN_TOKEN are available on Project Settings page on scan.coverity.com
ERROR: COVERITY_SCAN_TOKEN must be set
Running coverity script failed, ignoring.