Altair static resources not served if app context is not root

Similar to #125, Altair static resources are not served if app context is changed to something else. The culprit is [this line](https://github.com/graphql-java-kickstart/graphql-spring-boot/blob/master/altair-spring-boot-autoconfigure/src/main/java/com/oembedler/moon/altair/boot/AltairController.java#L113), the `String.join(...)` should've been `StringUtils.join(...)`.