Automatically rerun failed scenarios

The following changes need to be made:

In the .travis.yml file, the - composer behat line in the script: section needs to be changed into the following:
```
- composer behat || composer behat-rerun
```
In the composer-json file, the requirement on wp-cli/wp-cli-tests needs to be adapted to require at least v2.0.7:
```
"wp-cli/wp-cli-tests": "^2.0.7"
```
In the composer-json file, the "scripts" section needs to be extended. Immediately after the line "behat": "run-behat-tests",, the following line needs to be inserted:
```
"behat-rerun": "rerun-behat-tests",
```

Here's an example of how this should look like:

wp-cli / scaffold-command