Change paths determination for CLI

Just a slight modification to how paths are determined before loading a file. (in the node/cli env)

If `env.paths` does not exist, it introduces `undefined` into the array. This breaks in node v0.10+ by [throwing an exception](http://nodejs.org/docs/latest/api/path.html#path_path_join_path1_path2) when `path.join` is called. Secondly, this also checks for whether or not `.` has already been added to the list before appending it.

Currently, I always have to make sure `paths` is defined in my LESS options, this eliminates that necessity. (since the defaults here are reasonable)