Add JDK 10 Early-Access to Travis CI build matrix

jdk-10-ea+30 from http://jdk.java.net/10/

.travis.yml

@@ -1,5 +1,4 @@
 language: java
-jdk: oraclejdk9
 
 sudo: false
 dist: trusty
@@ -9,10 +8,18 @@ branches:
   except:
     - /^r\d.*$/
 
-# Don't let Travis CI execute './gradlew assemble' by default
-install:
-# Display Gradle, Groovy, JVM and other versions
-  - ./gradlew -version
+matrix:
+  include:
+# Java 9 (openjdk-9.0.1 fails with java.security.InvalidAlgorithmParameterException: the trustAnchors parameter must be non-empty)
+    - jdk: oraclejdk9
+      env: JDK_RELEASE='JDK 9'
+      install: echo "Don't let Travis CI execute './gradlew assemble' by default"
+# Java 10
+    - env: JDK_RELEASE='JDK 10 Early-Access'
+      install: . ./src/install/install-jdk-10.sh
 
 script:
+# Display Gradle, Groovy, JVM and other versions
+  - ./gradlew -version
+# Build JUnit 5
   - ./gradlew build --scan --stacktrace

src/install/install-jdk-10.sh

@@ -0,0 +1,14 @@
+#!/bin/bash
+set -e
+
+JDK_FEATURE=10
+JDK_BUILD=30
+JDK_ARCHIVE=jdk-${JDK_FEATURE}-ea+${JDK_BUILD}_linux-x64_bin.tar.gz
+
+cd ~
+wget http://download.java.net/java/jdk${JDK_FEATURE}/archive/${JDK_BUILD}/binaries/${JDK_ARCHIVE}
+tar -xzf ${JDK_ARCHIVE}
+export JAVA_HOME=~/jdk-${JDK_FEATURE}
+export PATH=${JAVA_HOME}/bin:$PATH
+cd -
+java --version

src/install/install-jdk-9.sh

@@ -0,0 +1,13 @@
+#!/bin/bash
+set -e
+
+JDK_FEATURE=9.0.1
+JDK_ARCHIVE=openjdk-${JDK_FEATURE}_linux-x64_bin.tar.gz
+
+cd ~
+wget http://download.java.net/java/GA/jdk9/${JDK_FEATURE}/binaries/${JDK_ARCHIVE}
+tar -xzf ${JDK_ARCHIVE}
+export JAVA_HOME=~/jdk-${JDK_FEATURE}
+export PATH=${JAVA_HOME}/bin:$PATH
+cd -
+java --version