oraclejdk8 for Travis build

Attempt to avoid problems when building in Travis for the itemis nexus
repository

[ERROR] Internal error: java.lang.RuntimeException:
org.apache.maven.MavenExecutionException: Could not setup plugin
ClassRealm: Plugin org.fornax.toolsupport:fornax-oaw-m2-plugin:3.4.0 or
one of its dependencies could not be resolved: Failed to read artifact
descriptor for org.fornax.toolsupport:fornax-oaw-m2-plugin:jar:3.4.0:
Could not transfer artifact
org.fornax.toolsupport:fornax-oaw-m2-plugin:pom:3.4.0 from/to
fornax.plugin.repository
(https://projects.itemis.de/nexus/content/groups/public/):
java.lang.RuntimeException: Could not generate DH keypair: Prime size
must be multiple of 64, and can only range from 512 to 1024 (inclusive)

http://stackoverflow.com/questions/10687200/java-7-and-could-not-generate-dh-keypair#comment42160155_16687105

Change-Id: I2e18b375f1c73f462e201cddf341be7fd2298fb1