v2.7.0

Enhancements:

• Add URL to invalid URL exception message for easier debugging (#122).

Bug fixes:

• When serializing to a CSV file, steps after the initial one should append to the existing CSV file rather than overwriting it (#123).

Internal changes:

• Switch from Travis CI to GitHub Actions (#117).
• Upgrade PHPUnit (#118).
• Enable nightly builds (#120).
• Change build so GitHub Actions run in forks as well without requiring secrets to be configured (#124).