Use OpenJDK in TravisCI config instead of OracleJDK

OpenJDK is recommended at TravisCI these days.

OracleJDK 8 is no more available for any version of Ubuntu there.

Manually install OpenJDK from Ubuntu repositories (i.e. apt install openjdk-11-jdk) because the pre-installed one from TravisCI is too old, e.g. JDK 11.0.2 instead of 11.0.10.

Add a job to build and test on ARM64 - a more complete version of https://github.com/apache/ranger/pull/91.