.travis.yml: Split the script command [skip ci]

.travis.yml

 ---
 language: ruby
 bundler_args: --without development
-script: "bundle exec rake validate && bundle exec rake lint && bundle exec rake spec SPEC_OPTS='--format documentation' && bundle exec rake metadata"
+script: ["bundle exec rake validate", "bundle exec rake lint", "bundle exec rake spec SPEC_OPTS='--format documentation'", "bundle exec rake metadata"]
 matrix:
   fast_finish: true
   include: