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

Re-producing the warning message:

```
$ sudo update-java-alternatives -s java-8-oracle ; sbt -v about
# Executing command line:
java
-Xms1024m
-Xmx1024m
-XX:MaxPermSize=256m
-XX:ReservedCodeCacheSize=128m
-jar
/usr/share/sbt-launcher-packaging/bin/sbt-launch.jar
about

Java HotSpot(TM) 64-Bit Server VM warning: ignoring option MaxPermSize=256m; support was removed in 8.0
```