Update CHANGELOG.md for 9.1.8
Update CHANGELOG.md for 9.1.8

[ci skip]
dplarson/gitlabhq,stoplightio/gitlabhq,stoplightio/gitlabhq,dplarson/gitlabhq,dreampet/gitlab,axilleas/gitlabhq,mmkassem/gitlabhq,t-zuehlsdorff/gitlabhq,dplarson/gitlabhq,mmkassem/gitlabhq,axilleas/gitlabhq,mmkassem/gitlabhq,iiet/iiet-git,jirutka/gitlabhq,jirutka/gitlabhq,jirutka/gitlabhq,dreampet/gitlab,dreampet/gitlab,axilleas/gitlabhq,iiet/iiet-git,stoplightio/gitlabhq,iiet/iiet-git,t-zuehlsdorff/gitlabhq,iiet/iiet-git,t-zuehlsdorff/gitlabhq,axilleas/gitlabhq,stoplightio/gitlabhq,dreampet/gitlab,t-zuehlsdorff/gitlabhq,mmkassem/gitlabhq,jirutka/gitlabhq,dplarson/gitlabhq