Fix class format error in enhancer

If I understood thing correctly, abstract classes do not need enhancement.

Fix
Caused by: java.lang.ClassFormatError: Code attribute in native or abstract methods in class file fr/lactalis/gmp/app/module/installation/entity/EvenementDispositif$Enhanced
at java.lang.ClassLoader.defineClass1(Native Method) [rt.jar:1.7.0_15]
at java.lang.ClassLoader.defineClass(Unknown Source) [rt.jar:1.7.0_15]
at java.lang.ClassLoader.defineClass(Unknown Source) [rt.jar:1.7.0_15]
... 21 more

Described in https://github.com/BatooOrg/BatooJPA/issues/143