(issue/42) fix error when version isn't specified

Error message when a gem version isn't specified and a Gemfile.lock
isn't present referred to a non-existant variable, so it would print a
stack trace instead of the error message. This commit refers to the
correct variable.
Merge pull request #43 from MikaelSmith/issue/42-mirror-error

(issue/42) fix error when version isn't specified