add java dependency to BEAST 2 version.xml

[walterxie]

@rbouckaert @alexeid @tgvaughan Since we have multiple versions of BEAST 2 working in different versions of Java, we should record this dependency somewhere. The best place, I think, is version.xml.

Will it break anything, if we add this line inside <addon name='BEAST' version='2.5.0' ...? <depends on='Java' atleast='1.8.0'/>

[walterxie]

This can be used for Remco's code static protected boolean javaVersionCheck(String app) to find the minimum working version of Java.

[rbouckaert]

BEAST itself requires Java 8, but its packages may require higher versions, so it makes sense to add Java version dependencies to version.xml files for packages. However, I do no envision BEAST being anything but Java 8 compliant for the near future, so not quite sure how much it helps for BEAST itself.

[walterxie]

@rbouckaert I think it will be much useful to inform BEAST users. At the moment, I cannot find "BEAST2 requires Java 8" from our website (I am going to add it soon). If we add this information in version.xml, we could make it available after I implement the script to auto-update features table. On the other hand, if you make your javaVersionCheck(String app) working for BEAST 2 version.xml, when we are moving to Java 1.9, you would just simply update the version.xml instead of changing the code again.

I will create the milestone 2.6 and try to add this after 2.5 release.

[walterxie]

related to #770