JAVA_OPTS is not honored

The doc tells:

> JVM Flags
> Ammonite also supports the JAVA_OPTS environment variable for passing arguments to the JVM that it runs  inside, e.g. you can pass in a custom memory limit via

By reverse engineering I've seen that I must use in order to have a 2Gb heap size:

```
./myScript.sc -J-Xmx2g
```

instead of:
```
JAVA_OPTS=-Xmx2g ./myscript.sc
```