Update JavaWriter to the latest.

Specifically this fixes a problem with handling fully-qualified class names which are nested under 'java.lang' but not directly in the 'java.lang' package (e.g., 'java.lang.ref.WeakReference').

JavaWriter already has tests for this: https://github.com/square/javawriter/blob/master/src/test/java/com/squareup/javawriter/JavaWriterTest.java#L854-L864

Closes #420.