Fixed regression when executing script files

The Groovy version of DslScriptLoader causes a regression when executing script files with a name that is not a valid Java class name, see [GROOVY-4020](https://jira.codehaus.org/browse/GROOVY-4020). E.g. when running a script called `jobs-blog.groovy` the following exception is thrown:

```
java.lang.ClassFormatError: Illegal class name "jobs-blog$run" in class file jobs-blog$run
```

One would assume that this should also be a problem in the Java version of DslScriptLoader, but for some reason it isn't.

I did this in two commits to allow Git to maintain the history of the file.