Add explicit reference to rouge 1.10.1

We are relying on rouge 1.10.1 but nothing is referencing to this version and it is easy to lose this dependence. Previously, it was updated directly in Gemfile.lock which is not reliable way :)

https://gitlab.com/gitlab-org/gitlab-ce/commit/e20529cec4f6de0c2c1589da99abb8714ea1c1c1

gollum-lib has a dependence rouge 1.9 which is old for us.