Allow relative directories as gem names in bundle gem command

I often try to do `bundle gem .` once inside the directory I set up for
the gem. This do creates boilerplate in the current directory, however
it assumes the name is named `.`.

With this patch you can do `bundle gem .` and it will scaffold the game
with the current folder name. It even goes a bit beyond that and allows
any relative path to be a valid gem target.
Merge pull request #3281 from gsamokovarov/relative-gem-names

Allow relative directories as gem names in bundle gem command