Look up playerglobal.swc location in flex config xml

I'm working on a project that uses an older version of the Flex SDK. In this SDK the playerglobal.swc is located in a directory:
```
frameworks/libs/player/{targetPlayerMajorVersion}/playerglobal.swc
```

So the hardcoded path in CommandLineInstruction.groovy gives problems. This change looks up the path in the flex config xml.

I'm not particularly familiar with flex so I'm not sure if this is the right fix. Some way to configure this path would be nice though.