Improve CI/QA process

[jrfnl]

Also see PRs #55, #57, #61 and #62.

I'd like to suggest expanding the automated CI/QA checks done via Travis to prevent the above mentioned issues and/or get earlier warning about them.

Suggestions for things to add to the Travis build:

• Linting the code against all supported PHP versions.
• An automated scan of the code with PHPCompatibility
• Security check of the Composer dependencies.
• Validation of the Composer file.

[gmazzap]

Linting the code against all supported PHP versions.

This has been extensively discussed in #65. Summary: I don't think this is necessary.

An automated scan of the code with PHPCompatibility

Nice to have.

Security check of the Composer dependencies

As a development-only library, so nothing that should leave local environment, I don't think this is much relevant. It does not hurt though, even because it does not need any additional code, just requiring a no-code meta package, so why not.

Validation of the Composer file.

Not necessary. Dependencies are installed in CI, and Composer does itself a validation of composer.json (exactly same validation that could be done via composer validate) before installing dependencies.
Running validation separately would mean to do a double validation when the file is valid (most of the times) and will only cause an error to be discovered just a few milliseconds earlier. No real benefit and unnecessary slow down of build time (tiny, but still).

[gmazzap]

With merged #66 this can be closed. Thanks again @jrfnl