[ci] Switch to OpenJDK 10

See https://sormuras.github.io/blog/2018-03-20-jdk-matrix.html

.travis.yml

@@ -1,16 +1,11 @@
 dist: trusty
 sudo: false
 addons:
-  apt:
-    packages:
-      - oracle-java9-installer
   ssh_known_hosts:
     - web.sourceforge.net
 
 language: java
 
-jdk: oraclejdk9
-
 env:
   global:
   - secure: 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
@@ -31,9 +26,11 @@ matrix:
   fast_finish: true
 
 before_install:
+  - wget https://github.com/sormuras/bach/raw/master/install-jdk.sh
   - bash .travis/setup-secrets.sh
   - bash .travis/configure-maven.sh
-install: true
+# Install OpenJDK 10 - see https://sormuras.github.io/blog/2018-03-20-jdk-matrix.html
+install: . ./install-jdk.sh -F 10 -L GPL
 before_script: true
 script: source .travis/build-$BUILD.sh
 after_success: true