Thymeleaf Layout Dialect 2.3.0

• Verification that the layout dialect plays well with Java 9 and Spring 5, involved a patch upgrade of Groovy to 2.4.13 (#161)
• Contribution from @Vineg to add a layout:collect/data-layout-collect processor that accumulates encountered fragments of the same name. Documentation is still pending, but eager devs can take a look at the PR for more details (#166)
• A few tool updates for linting, automated testing, and code coverage w/ Travis CI (included dropping the JDK7 build as I could no longer get a supported configuration working on Travis, but the layout dialect still aims to support Java 7 for the remainder of the 2.x releases).