Maven now tests if files generated by the archetypes do actually build
[FLINK-1042] Changed ClassLoader field to be transient.

Serialization of the `RuntimeStatefulSerializerFactory` fails because the field `ClassLoader loader` is not serializable. The fix makes the field `transient`.

Author: Fabian Hueske <fhueske@apache.org>

Closes #92 from fhueske/StatefulSerializerFactoryFix and squashes the following commits:

c01cc27 [Fabian Hueske] [FLINK-1042] Changed ClassLoader field to be transient.
Remove field0 from CoGroup and Join Operator

These were added before to force the user to specify at least one key
field. Now, the user is also forced to specify at least one key because
otherwise the call would be ambiguous because of tuple key fields and
expression keys.