Make it more clear that Java 8 is required (openhab#766)

* Make it more clear that Java 8 is required

Reword to make it more clear that Java 8 is required and all of the newer versions of Java are not currently supported.

Addresses openhab#765

signed off by Rich Koshak <rlkoshak@gmail.com>

installation/index.md

@@ -37,7 +37,7 @@ Please check the menu to the left for all available options.
 
 ## Prerequisites
 
-Make sure that you have an up to date Java platform installed on your host system.
+Make sure that you have an up to date Java 8 platform installed on your host system.
 Zulu is currently the recommended Java platform for openHAB.
 [Download](https://zulu.org/download/?show=all) and [installation](http://zulu.org/zuludocs-folder/Content/ZuluInstallationGuide/InstallingZulu/InstallZulu.htm) instructions can be found on Azul Systems' Zulu website.
 Oracle Java is also suitable for most configurations but licencing restrictions may apply.
@@ -49,11 +49,12 @@ OpenJDK may also be used, but it has some [known limitations](https://community.
 | [Oracle Java](https://java.com/en/) | Full openHAB support on all platforms | [Licensing restrictions](https://blog.takipi.com/running-java-on-docker-youre-breaking-the-law/), manual installation required on many Linux systems: [Ubuntu](https://help.ubuntu.com/community/Java), [Mint](https://community.linuxmint.com/tutorial/view/1091), and [Debian](https://wiki.debian.org/Java/Sun) are some examples |
 | [OpenJDK](http://openjdk.java.net) | Supported by many Linux distributions, packages [typically available](http://openjdk.java.net/install/index.html) | [Performance issues](https://github.com/openhab/openhab-distro/issues/10#issuecomment-223786506) on ARM platforms, [compatibility issues](https://community.openhab.org/t/openhab-is-offline-message-fixed/17441/8) with certain bindings and certificates |
 
+Please download and install the Java 8 version of the JVM. 
+openHAB is known not to work on Java 9 and Java 10.
+
 Please use the **32-bit version** of the JVM for ARM platforms, even on 64-bit operating systems.
 Serial connections won't work with a 64-bit JVM, preventing bindings like Z-Wave from functioning.
 
-As for now, a pre-release **Java 9** installation is not recommended and is not yet supported by openHAB 2.
-
 For best compatibility, namely with the openHAB Cloud service [myopenhab.org](http://www.myopenhab.org), the minimum recommended Java 8 revision is "101".
 Attention: Most package managers serve an **older revision**.
 Check your current Java version by opening a command line console and typing `java -version`: