make code compiles with jdk 6

This pull request closes #58 it changes the code so that it compiles with jdk 6 instead of 7. 
> Facepalm for me. :)

No problem, I did that mistake several times. The tests still needs the jdk 7. So one has to validate the code with the jdk 7. But before I always try to compile it with jdk 6. 
One could solve it with restructuring the tests. It's possible to have JUnit tests at the same place but move the files which are scanned by spoon into the resource folder. Maven wouldn't compile them anymore and so the scanned files could use java 7 or 8.. But the used jdk can be still 6. One could give it a try with another pull request. 

@monperrus @pschichtel Do you have any suggestions to solve the generic method type thing without removing it?