Project won't start in m2ee tools after migration to Mx7.3

After updating our project to Mx 7.3.0 we get the following exception during startup.

Exception in thread "main" java.lang.UnsupportedClassVersionError: com/mendix/http/HttpConfiguration : Unsupported major.minor version 52.0

The m2ee-tools version is 7.0.1 

I already looked into our userlib folder to see if we have a lib that has this class somewhere but to no avail.