Automatically rerun failed scenarios

[schlessera]

The following changes need to be made:

1. 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

2. 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"

3. 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",

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