fix a raising of NotImplemented

It may not have caused any issues, but changing this from raising
NotImplemented to a NotImplementedError. Explanation about why this is
important at:
http://mouadino.appspot.com/notimpelementederror-vs-notimplemented-in-python/

gramps/plugins/lib/libgedcom.py

@@ -1238,7 +1238,7 @@ class BaseReader(object):
         self.ifile.seek(0)
 
     def readline(self):
-        raise NotImplemented
+        raise NotImplementedError()
 
     def report_error(self, problem, line):
         line = line.rstrip('\n\r')