Fix require compiler/iseq to use path on gem.

I just downloaded current rubinius and after building, tried to execute `rbx compile -P hello.rb` 

Failed with 

```
    no such file to load -- compiler/iseq (LoadError)
```

this path was used when the compiler files were inside rubinius codebase, now it's distributed as a separate gem and the path has changed to rubinius/compiler/iseq. This pull request fixes this tiny error. 

Also needed to apply rubinius/rubinius-compiler#1 for it to work.