CI: switch to GH Actions

[jrfnl]

Fixes #15

Composer/Ant: move the build scripts to the Composer configuration

As this is a PHP project and the build steps are relatively simple, it will make it more intuitive for contributors to use Composer scripts to run CI checks, than to have to install Ant for this.

This commit creates the same build/CI scripts as were previously available via Ant in the Composer configuration file.

Notes:

• The scripts are set up to respect the PHP version used by Composer (instead of a system default).
• The scripts are set up to work equally well on Linux/Windows/Mac.
• The only two differences are:
  • The mkdir and file deletions won't automatically be done anymore.
    Generally speaking, the mkdir should only need to be run once anyway and the file deletes should not be necessary as the log files will just be overwritten by the tools, so I deemed this an acceptable trade-off.
  • The colors argument won't be set anymore.
    For most tools, this means, the tool will default to automatically determining whether the CLI used supports colourized output.
    As the determination of this in most tools has become pretty good, I, again, deemed this an acceptable trade-off.

CI: switch to GitHub Actions - step 1: code style sniffing

This commit:

• Adds a GH Actions workflow for the code style CI check which was previously run on Travis.
• Removes that part of the .travis.yml configuration.
• Updates the .gitattributes file.
• Adds a separate badge for the CS workflow to the README.

CI: switch to GitHub Actions - step 2: linting and tests

This commit:

• Adds a GH Actions workflow for the PHP lint and unit test CI checks which were previously run on Travis.
• Removes the, now redundant, .travis.yml configuration.
• Updates the .gitattributes file.
• Updates the "Build Status" badge in the Readme to use the results from the GH Test Actions runs.

[grogy]

Thank you, it looks very good 👍