Fix recursion conflict with RCP rcp_set_customer_trialing_flag (#26).

[rvdsteege]

Fix #26 by changing the priority with which we hook into rcp_transition_membership_status.

[remcotolsma]

Is it an idea to also add a documentation block above this that explains the importance of priority 9? With a link to issue:

• Infinite loop when activating subscription with past expiry date #26

[rvdsteege]

Is it an idea to also add a documentation block above this that explains the importance of priority 9? With a link to issue:

• Infinite loop when activating subscription with past expiry date #26

Added in 38fa944.