Update CHANGELOG.md for 9.1.5
Update CHANGELOG.md for 9.1.5

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