COMP: operator< must be const member function

When using clang 5 w/ c++11 the correct function was not being found.

Change-Id: I951b2252070effd31bbc30585bd34aa95a6ba59f
Merge topic 'SIMPLEITK-525_EnableDeletionOfCommandProcessExists' into next

9f88dc21 COMP: operator< must be const member function