[FEATURE] Validate the composer.json on Travis (#245)

Author: oliverklee

.travis.yml

@@ -51,6 +51,11 @@ install:
   composer show;
 
 script:
+- >
+  echo;
+  echo "Validating the composer.json";
+  composer validate --no-check-all --no-check-lock --strict;
+
 - >
   echo;
   echo "Linting all PHP files";