Add Java 9 to Travis

[ashawley]

Fix #173 based on @xuwei-k's work in #179

[ashawley]

Looks like the modules/ trick for Java 9 doesn't work with 2.11.12:

> ++2.11.12
[info] Setting version to 2.11.12
[info] Reapplying settings...
[info] Set current project to root (in build file:scala-xml/)
> show xmlJVM/apiMappings
[info] Map(~/.ivy2/cache/org.scala-lang/scala-library/jars/scala-library-2.11.12.jar -> http://www.scala-lang.org/api/2.11.12/, /modules/java.base -> http://docs.oracle.com/javase/9/docs/api, /modules/java.xml -> http://docs.oracle.com/javase/9/docs/api)
> xmlJVM/doc
[info] Updating {file:scala-xml/}xmlJVM...
[info] Done updating.
[info] Main Scala API documentation to scala-xml/jvm/target/scala-2.11/api...
model contains 147 documentable templates
[warn] scala-xml/shared/src/main/scala/scala/xml/parsing/FactoryAdapter.scala:164: Could not find any member to link for "org.xml.sax.SAXException".
[warn]   /**
[warn]   ^
[warn] scala-xml/shared/src/main/scala/scala/xml/include/sax/EncodingHeuristics.scala:38: Could not find any member to link for "java.io.IOException".
[warn]   /**
[warn]   ^
[warn] scala-xml/shared/src/main/scala/scala/xml/package.scala:11: Could not find any member to link for "javax.xml.parsers.SAXParser".
[warn] /**
[warn] ^
[warn] scala-xml/shared/src/main/scala/scala/xml/include/XIncludeException.scala:13: Could not find any member to link for "java.lang.Throwable#getMessage".
[warn] /**
[warn] ^
[warn] four warnings found
[info] Main Scala API documentation successful.

[ashawley]

Today, sbt 0.13.17 is released, so the build works with Java 9.