Fix terminology used in BuildTrigger conditions

If the result needs to be SUCCESS, it needs to be called 'stable',
not 'successful' -- the latter is UNSTABLE or better.

References:
- PermalinkProjectAction (lastStableBuild for SUCCESS,
  lastSuccessfulBuild for UNSTABLE or better)
- Last Successful list view column (includes unstable builds),
  Last Stable column (doesn't)
Merge pull request #1152 from daniel-beck/BuildTrigger_terminology

Fix terminology used in BuildTrigger conditions