[ZEPPELIN-5002] Update JDK version in documentation
[ZEPPELIN-5002] Update JDK version in documentation

### What is this PR for?
Java 8 is required to building from source.

In 'install' page, it is 1.8 (151+)
http://zeppelin.apache.org/docs/0.9.0-preview2/quickstart/install.html

but in 'build' page, it's 1.7
http://zeppelin.apache.org/docs/0.9.0-preview2/setup/basics/how_to_build.html#how-to-build-zeppelin-from-source

### What type of PR is it?
Bug Fix

### Todos
* [ ] - Task

### What is the Jira issue?
https://issues.apache.org/jira/browse/ZEPPELIN-5002

### How should this be tested?
* First time? Setup Travis CI as described on https://zeppelin.apache.org/contribution/contributions.html#continuous-integration
* Strongly recommended: add automated unit tests for any new or changed behavior
* Outline any manual steps to test the PR here.

### Screenshots (if appropriate)

### Questions:
* Does the licenses files need update?
* Is there breaking changes for older versions?
* Does this needs documentation?

Author: Philipp Dallig <philipp.dallig@gmail.com>
Author: Yurim Jin <jayjinjay@gmail.com>

Closes #3880 from milooy/feat/ZEPPELIN-5002 and squashes the following commits:

2c423ab8a [Philipp Dallig] Update how_to_build.md
94ec52849 [Yurim Jin] [ZEPPELIN-5002] Update JDK version in documentation

hammertank/zeppelin,prabhjyotsingh/zeppelin,joroKr21/incubator-zeppelin,prabhjyotsingh/incubator-zeppelin,jongyoul/incubator-zeppelin,jongyoul/incubator-zeppelin,apache/incubator-zeppelin,hammertank/zeppelin,jongyoul/zeppelin,zjffdu/zeppelin,VipinRathor/zeppelin,zjffdu/zeppelin,apache/zeppelin,prabhjyotsingh/zeppelin,apache/zeppelin,VipinRathor/zeppelin,apache/incubator-zeppelin,fogbeam/zeppelin_mirror,hammertank/zeppelin,zjffdu/zeppelin,prabhjyotsingh/zeppelin,joroKr21/incubator-zeppelin,zjffdu/zeppelin,VipinRathor/zeppelin,apache/zeppelin,prabhjyotsingh/zeppelin,fogbeam/zeppelin_mirror,apache/zeppelin,fogbeam/zeppelin_mirror,hammertank/zeppelin,joroKr21/incubator-zeppelin,jongyoul/zeppelin,prabhjyotsingh/incubator-zeppelin,zjffdu/zeppelin,VipinRathor/zeppelin,jongyoul/incubator-zeppelin,jongyoul/zeppelin,apache/incubator-zeppelin,jongyoul/incubator-zeppelin,hammertank/zeppelin,fogbeam/zeppelin_mirror,fogbeam/zeppelin_mirror,prabhjyotsingh/incubator-zeppelin,apache/incubator-zeppelin,VipinRathor/zeppelin,apache/incubator-zeppelin,joroKr21/incubator-zeppelin,apache/zeppelin,apache/incubator-zeppelin,apache/zeppelin,joroKr21/incubator-zeppelin,jongyoul/zeppelin,zjffdu/zeppelin,hammertank/zeppelin,jongyoul/zeppelin,jongyoul/incubator-zeppelin,prabhjyotsingh/incubator-zeppelin,jongyoul/zeppelin,prabhjyotsingh/zeppelin,joroKr21/incubator-zeppelin,hammertank/zeppelin,VipinRathor/zeppelin,prabhjyotsingh/incubator-zeppelin,prabhjyotsingh/zeppelin,zjffdu/zeppelin,VipinRathor/zeppelin,jongyoul/zeppelin,joroKr21/incubator-zeppelin,apache/zeppelin,prabhjyotsingh/incubator-zeppelin,prabhjyotsingh/zeppelin,joroKr21/incubator-zeppelin,fogbeam/zeppelin_mirror,fogbeam/zeppelin_mirror,jongyoul/incubator-zeppelin