Enhancement: Validate composer on Travis

[localheinz]

This PR

• [x] validates composer.json and composer.lock on Travis
• [x] updates composer.lock by running composer update --lock

💁 Running

$ composer validate

locally yields

./composer.json is valid
The lock file is not up to date with the latest changes in composer.json, it is recommended that you run `composer update`.

[weierophinney]

Thanks, @localheinz !