Javadoc html warning when running against JDK 10

[jeremydouglass]

Processing library template projects are written against Java 1.8 -- when compiling against JDK 10 javadoc gives a warning about the html format not being specified.

  [javadoc] Generating Javadoc
  [javadoc] Javadoc execution
  [...]
  [javadoc] Constructing Javadoc information...
  [javadoc] javadoc: warning - You have not specified the version of HTML to use.
  [javadoc] The default is currently HTML 4.01, but this will change to HTML5
  [javadoc] in a future release. To suppress this warning, please specify the
  [javadoc] version of HTML used in your documentation comments and to be
  [javadoc] generated by this doclet, using the -html4 or -html5 options.
  [javadoc] Standard Doclet version 10.0.1 

This happens when building on OS X 10.12.6 (default JDK 10). There is a related issue here that lists a fix (for gradle, not ant): https://github.com/gradle/gradle/issues/5891

That fix:

1. check the java version
2. set the html flag if it the version > 9.