Adding -jdk15 to version

[Karl Heinz Marbaise]

Based on the idea using it as dependencies etc. the idea add something to the version to mark it for particular JDK usage is a bad practice. For such purpose a classifier exists... I would suggest to change that.

[Jordan Zimmerman]

Thanks - I'll look into that. However, I'm probably going to drop Java 15 support soon. See here: #55