Added gitlab CI. Updated completions, readme.

Added probe to the completion list.
3 jobs for system_tests in 9 minutes and 46 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Build And Test
passed #245259
ligo-channel-completion:buster

00:01:05

passed #245261
ligo-channel-completion:jessie

00:09:46

passed #245260
ligo-channel-completion:stretch

00:00:58