Fix case when model has template field which is ForeignKey

[Lagovas]

When model has field with name "template" and has ForeignKey type next code

def _get_compare_func(suffix):
            # logger.debug("func_name: %s", func_name)
            func_name = "compare_%s" % suffix
            if hasattr(self, func_name):
                func = getattr(self, func_name)
                if callable(func):
                    return func

get compare_template field from VersionAdmin class which is str object, not callable.

[coveralls]

Coverage increased (+0.02%) to 86.772% when pulling 7c01ab5 on Lagovas:master into 265f4e6 on jedie:master.

[jedie]

Sorry for the late response time... This pull request is no release with v0.7.3 ;)

Thanks for contribution!