Remove the framework/CMS rulesets

[jrfnl]

... in favour of having them in their own repositories.

⚠️ This PR should only be merged once PR #689, PR PHPCompatibility/PHPCompatibilityJoomla#1 and PR PHPCompatibility/PHPCompatibilityWP#1 have been merged.

Notes:

• Removed the framework specific rulesets from this repo.

• Remove the framework ruleset specific build tests from the .travis.yml script.

• Adjusted the information in the Readme to point to the dedicated repos and add information about the PHPCompatibilityAll repo.

• Adjusted the information in the Contributing.md file.

  I've chosen to leave the general framework/CMS ruleset guidelines in the Contributing file so they are available to people who want to add a new ruleset for a framework/CMS not yet covered.

Related to #688

[jrfnl]

Workflow to get everything merged & builds to pass:

• Merge Update texts for move to GH organization #689 which includes the change in the composer package name.
• Edit the package on Packagist to point to the repo in the new organisation.
• Restart the Travis builds for Initial commit / v 1.0.0 PHPCompatibilityJoomla#1 and Initial commit / v 1.0.0 PHPCompatibilityWP#1
• Once the builds have passed, merge these PRs and tag the 1.0.0 releases.
• Submit the PHPCompatibilityJoomla and PHPCompatibilityWP packages to Packagist.
• Restart the Travis builds for Initial commit / v 1.0.0 PHPCompatibilityAll#1
• Once the build has passed, merge the Initial commit / v 1.0.0 PHPCompatibilityAll#1 PR and tag the 1.0.0 release.
• Submit the PHPCompatibilityAll package to Packagist.
• Merge this PR.