Change dokka outputFormat to "javadoc"

[pbcornell]

As identified in #130:

• javadoc.io doesn't recognize dokka's "html" outputFormat
• Kotlin's dokka tool fails on JDK 10 or higher when the "javadoc" outputFormat is specified (see dokka/294)

In the interim, the release process requires manually changing dokka's outputFormat to "html" and building with JDK 8.

Once dokka/294 is addressed (or javadoc.io recognizes dokka's "html" output), we should remove this manual step from the release process (by either setting dokka's outputFormat to "javadoc" in the repository, or successfully publishing dokka's "html" output to javadoc.io).