Update windup startup script to not show warnings for java 8 runtime

If you run Windup with jvm 8, Windup will print out the warning about used jvm option which was removed. This is about setting MaxPermSize for jvm 7 where with default value it it usually will throw OOM.

Warning: JAVA_HOME environment variable is not set.
OpenJDK 64-Bit Server VM warning: ignoring option MaxPermSize=256m; support was removed in 8.0