Use the same JDK in docs and tests

Nobody has claimed this yet.

This is a follow-up to issue https://github.com/JanusGraph/janusgraph/issues/304 which made me realize this inconsistency.

The documentation in [`docs/intro.txt`](https://github.com/JanusGraph/janusgraph/blob/25b8f1080f7d4790b959cf6b7fe443cb68486386/docs/intro.txt#L128) says:

> JanusGraph requires Java 8 (Standard Edition). Oracle Java 8 is recommended.

However, our [Travis CI config](https://github.com/JanusGraph/janusgraph/blob/25b8f1080f7d4790b959cf6b7fe443cb68486386/.travis.yml#L5-L6) explicitly only tests with OpenJDK 8:

> ```yaml
> jdk:
> - openjdk8
> ```

Also, [`BUILDING.md`](https://github.com/JanusGraph/janusgraph/blob/4c8c414349bb2d5f68e8b26f641d63b1b79b8908/BUILDING.md#building-janusgraph) is confusing in a different way in that it does not specify which JDK distribution is preferred:

> ## Building JanusGraph
> Required:
>
> * Java 8
> * Maven 3

We need to reconcile this and be consistent everywhere, and test with the same JDK that we recommend that users should use.

Review docs/intro.txt, BUILDING.md, and .travis.yml first, comparing the Java and JDK distribution guidance in each file. Determine the single recommended JDK and update the documentation and Travis CI configuration so they agree; done means the stated requirement and CI test environment are consistent.

Written by the indexing model from the issue text.