Rename GreenMaster to PassingBuild

Builds off of !585 (merged) to allow finding green builds for anything, not just master.

release-tools rs-green-anything % CI=true be rake 'passing_build:all[11-10-stable]'
--> Searching for passing build on gitlab-org/gitlab-ee@11-10-stable-ee...
    Found at f78d924bcb35678a0fdc800e6479ba650b855532!
      GITALY_SERVER_VERSION: 1.33.0
      GITLAB_PAGES_VERSION: 1.5.0
      GITLAB_SHELL_VERSION: 9.0.0
      GITLAB_WORKHORSE_VERSION: 8.5.1
--> Searching for passing build on gitlab-org/gitlab-ce@11-10-stable...
    Found at 2d82633fdd95e0b7b1ea79bc12514ab4f6e71a88!
      GITALY_SERVER_VERSION: 1.33.0
      GITLAB_PAGES_VERSION: 1.5.0
      GITLAB_SHELL_VERSION: 9.0.0
      GITLAB_WORKHORSE_VERSION: 8.5.1

release-tools rs-green-anything % CI=true be rake 'passing_build:all[11-7-stable]'
--> Searching for passing build on gitlab-org/gitlab-ee@11-7-stable-ee...
    Found at e0376b06293b4471bcc76e6990cb66c14306cdab!
      GITALY_SERVER_VERSION: 1.12.2
      GITLAB_PAGES_VERSION: 1.3.1
      GITLAB_SHELL_VERSION: 8.4.4
      GITLAB_WORKHORSE_VERSION: 8.0.4
--> Searching for passing build on gitlab-org/gitlab-ce@11-7-stable...
    Found at 9d826aed9ff336a55f770c328dcf042e67823b48!
      GITALY_SERVER_VERSION: 1.12.2
      GITLAB_PAGES_VERSION: 1.3.1
      GITLAB_SHELL_VERSION: 8.4.4
      GITLAB_WORKHORSE_VERSION: 8.0.2