Prepare for Java 9 / Tapestry 5.5

[thackel]

Tapestry 5.4 still can't be run with Java 9 and will probably never run with Java 9.
Tapestry 5.5 seems to run with Java 9, but is also not available in a public snapshot repo.

This project is still depending on Tap 5.4, and might prevent an update to Tap5.5 and Java 9.

So early starting building vs 5.5 with Java 9 would be nice.