Upgrade the default Java Version to 8

With the release of Java 8u20 it is time to move to Java 8. This commit changes
the default Java version that will be used by the Buildpack to '1.8.0_+'.

[#70715740]
Merge branch 'cgfrost-70715740-upgrade-to-java-8'
Merge remote-tracking branch 'upstream/master'