[Java] Upgrade to JDK 1.8

I'm trying to use the parquet-arrow module from parquet-mr but I'm running into this error because the two projects use different major versions of Java:

  Cause: java.lang.ClassNotFoundException: org.apache.arrow.vector.types.pojo.ArrowType$Struct_

The struct is actually named `Struct` not `Struct_`.

This PR is to track work to upgrade to JDK 1.8

I should note that this is after the recent commit in parquet to upgrade to use arrow-0.8.0.