Jdk 10 support

[dgoetsch]

The project appears not to build with jdk 10.

FAILURE: Build failed with an exception.

* What went wrong:
Execution failed for task ':api:core:dokkaJavadoc'.
> com/sun/tools/doclets/formats/html/HtmlDoclet

* Try:
Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output. Run with --scan to get full insights.

* Get more help at https://help.gradle.org

BUILD FAILED in 23s
25 actionable tasks: 18 executed, 7 up-to-date

This issue has not been confirmed to be caused by jdk 10, but it is the current suspect. Works fine in java 9.

[dgoetsch]

Tied to a dokka issue..
Kotlin/dokka#294