[FEATURE] Validate the composer.json on Travis (#43)

Author: oliverklee

.travis.yml

@@ -53,6 +53,11 @@ before_script:
   mysql ${PHPLIST_DATABASE_NAME} < vendor/phplist/phplist4-core/Database/Schema.sql;
 
 script:
+- >
+  echo;
+  echo "Validating the composer.json";
+  composer validate --no-check-all --no-check-lock --strict;
+
 - >
   echo;
   echo "Linting all PHP files";