Fixes remote CSV filename from getting set to 'nil'.

Rails 4, Ruby 2.0.0, carrierwave, Heroku -- 
When creating a CSV with a URL  ( `Roo::CSV.new(remote_csv_url)`) , was throwing 
```ruby
TypeError (no implicit conversion of nil into String)
```
`make_tmpdir` was nil-ing the local `filename` by using both a local assignment and calling the filename instance var.