With Java 7 at end-of-life, we have updated the SiteWhere build process to use Java 8 when compiling the product releases. If you require Java 7 or earlier, it is still possible to do a custom build with an earlier compiler since none of the core code is using Java 8 constructs yet. The compiler version is specified in the root build.gradle file.
Kind regards,
The SiteWhere Development Team