Fix "string is locked" error when a spec fails.
Fix "string is locked" error when a spec fails.

When an integration spec failed, the GitshRunner thread wasn't correctly
cleaned up, which meant all subsequent integration specs would also fail.

jakirkham/gitsh,thoughtbot/gitsh,adrianheine/gitsh,thoughtbot/gitsh,jakirkham/gitsh,thoughtbot/gitsh,andybalaam/gitsh,andybalaam/gitsh,andybalaam/gitsh,adrianheine/gitsh,jakirkham/gitsh