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

Author: stof

.travis.yml

@@ -29,3 +29,5 @@ script:
   - ./bin/console lint:xliff translations
   # this checks that the application doesn't use dependencies with known security vulnerabilities
   - ./bin/console security:check --end-point=http://security.sensiolabs.org/check_lock
+  # This checks that the composer.json and composer.lock files are valid
+  - composer validate --strict