Bug fix for comments that do not have 'text' key

Because the tracker API does not return a hash with 'text' as a key for a comment with only a screen shot attached (without accompanying comment text), the #commit_comment? and #text method were blowing up. This change fixes that problem.
Merge pull request #2 from SethMusulin/patch-1

Bug fix for comments that do not have 'text' key