Updating Sparkle feed link in install_latest_oracle_java_8

Oracle apparently forgot to update the Sparkle feed for Java 8 Update
11 when they released Java 8 Update 20. Even though Java 8 Update 20 is
now out, the latest version available via Java 8 Update 11's Sparkle
feed is still is Java 8 Update 11:

https://javadl-esd-secure.oracle.com/update/mac/au-1.8.0_11.xml

To fix this, I've updated the install_latest_oracle_java_8 script to
use Java 8 Update 20's Sparkle feed:

https://javadl-esd-secure.oracle.com/update/mac/au-1.8.0_20.xml