Code coverage data not getting uploaded successfully

https://coveralls.io/github/killercup/cargo-edit would seem to suggest that coveralls has seen no coverage data since 2016.

We've currently got the following in `.travis.yml':

```yaml
- env COVERAGE_TESTING=1 travis-cargo --only stable coveralls --no-sudo
```

Perhaps the environment variable is responsible?

Start by inspecting .travis.yml and the travis-cargo coveralls command, then compare the configured coverage upload step with the linked Coveralls page. Done means the CI job successfully uploads coverage data and it appears on the Coveralls project page.

Written by the indexing model from the issue text.