consider a looser dependency on gollum-lib

related to gitlab-ce#18077 @connorshea

Now github-markup is already 1.4.0 and gollum-lib 4.1.0 wants ~> 1.3.3. I could update gollum-lib to 4.2.0 which will fix this conflict, but gitlab still wants ~> 4.1.0. If this is changed to ~> 4.1 that will allow me to update gollum-lib minor releases without breaking or patching gitlab.