Update INSTALL.md

Did a little research into what UseCompressedOops does.
Found this: http://javarevisited.blogspot.ca/2012/06/what-is-xxusecompressedoops-in-64-bit.html

According to that blog post, UseCompressedOops is enabled by default past Java 6 Update 18. Therefore having in the startup script of a server is redundant.
Merge pull request #669 from DZCreeper/patch-1

Update INSTALL.md