Replace slashes with dashes to avoid 422 errors

When gisting a file from a subdirectory, e.g.:

    $ gist dir/file.txt

the gist API will return a 422 error, because it doesn't like slashes in filenames.

This patch makes sure slashes are converted to dashes. I chose this solution over something like `File.basename` so that files with the same name from different subdirectories can still be gisted at the same time.