Mock service only exits on SIGINT, but not SIGKILL

[mboudreau]

I just upgrade pact-node to the latest version of the pact-mock-service, where it was using 0.7.2 before. It seems that the exit signal has changed since I used a to a SIGKILL on the process to close it and it worked, but now it has to be SIGINT. Personally, I think both should be supported by pact-mock_service since one is a graceful exit and the other is ungraceful, which should still work.