Add validation of the composer files on Travis
Add validation of the composer files on Travis

This performs validation of the composer metadata to ensure they are valid:

- composer.json does not contain any error
- composer.json does not trigger any warning (thanks to the strict mode)
- composer.lock is uptodate

TomasVotruba/symfony-demo,bocharsky-bw/symfony-demo,voronkovich/symfony-demo,javiereguiluz/symfony-demo,Codeception/symfony-demo,symfony/symfony-demo,voronkovich/symfony-demo,bocharsky-bw/symfony-demo,symfony/symfony-demo,javiereguiluz/symfony-demo,symfony/symfony-demo,Codeception/symfony-demo,javiereguiluz/symfony-demo,yceruto/symfony-demo,Codeception/symfony-demo,yceruto/symfony-demo,voronkovich/symfony-demo,bocharsky-bw/symfony-demo,TomasVotruba/symfony-demo,TomasVotruba/symfony-demo,yceruto/symfony-demo