Only calculate memory opts if we need them...

...don't bother if won't use them due to them already being defined.
Don't pass MaxPermSize to Java 8+

This change stops this annoying message being written to stderr (which can
cause IntelliJ to freak):

```
Java HotSpot(TM) 64-Bit Server VM warning: ignoring option MaxPermSize=256m; support was removed in 8.0
```

See also:

https://bugs.openjdk.java.net/browse/JDK-6964458
https://github.com/sbt/sbt-native-packager/issues/203
https://github.com/typesafehub/activator/issues/422
Merge pull request #66 from rtyley/only-specify-permgen-pre-java-8

Don't pass MaxPermSize to Java 8+