Add GitLab Mattermost license. Bump Mattermost version to 0.7.1
Add GitLab Mattermost license. Bump Mattermost version to 0.7.1

gitlabhq/omnibus-gitlab,copystudy/omnibus-gitlab,gitlabhq/omnibus-gitlab,gitlabhq/omnibus-gitlab,copystudy/omnibus-gitlab,gitlabhq/omnibus-gitlab,copystudy/omnibus-gitlab