Enable oraclejdk9, remove special java 8 installer

The installer workaround should no longer be necessary.

.travis.yml

@@ -1,21 +1,17 @@
 language: java
 jdk:
   - oraclejdk8
+  - oraclejdk9
 sudo: false
 
 env: DISPLAY=:99.0
 
 install: true
 
-# This line installs more modern Java 8 versions than Travis supports as of this writing
-# (8/2016). The version supported by Travis (8u31) was way out of date and crashing builds.
-# Source: https://github.com/travis-ci/travis-ci/issues/3259#issuecomment-130860338
-
-# also install a window manager, needed for SWT-based tests
+# install a window manager, needed for SWT-based tests
 addons:
   apt:
     packages:
-      - oracle-java8-installer
       - metacity
 
 # start metacity window manager before running tests