Regex used in rm-key command is too lax

Basically the issue is that 'gitlab-shell rm-key key-2' removes all keys that match "key-2" pattern.
"key-20", "key-25", etc.
This change makes the regex used in the sed command more strict.
Merge pull request #51 from max-krasnyansky/rm-key-fix

Regex used in rm-key command is too lax