Builds are failing because default jdk changed to openjdk11 on Travis

The default distribution on Travis recently changed from Trusy to Xenial. It appears that the default JDK also changed from JDK8 to JDK11, despite the doc says the default is openjdk8, it appears that it isn't correct (see related discussion)

Since Parquet still doesn't support Java 11 (PARQUET-1551), we should explicitly tell in Travis config which JDK to use, at lease as long as PARQUET-1551 is still open.