Filename can now be passed in without arguments

When I was trying to use the `filename` argument on the new heroku version (2.40.1), it seems to think that it is being passed a boolean instead of a string.
```bash
$ heroku config:push --filename configs/env/common.env

 !    Heroku client internal error.
 !    Search for help at: https://help.heroku.com
 !    Or report a bug at: https://github.com/heroku/heroku/issues/new

    Error:       undefined method `empty?' for true:TrueClass (NoMethodError)
    Backtrace:   /Users/saul/.heroku/plugins/heroku-config/lib/config/heroku/command/config.rb:48:in `filename'
                 /Users/saul/.heroku/plugins/heroku-config/lib/config/heroku/command/config.rb:41:in `push'
                 /Users/saul/.heroku/client/lib/heroku/command.rb:213:in `run'
                 /Users/saul/.heroku/client/lib/heroku/cli.rb:28:in `start'
                 /usr/bin/heroku:24:in `<main>'

    Command:     heroku config:push --filename configs/env/common.env
    Plugins:     heroku-config
                 heroku-pg-extras
                 heroku-pipeline
                 heroku-surrogate

    Version:     heroku-toolbelt/2.40.1 (x86_64-darwin10.8.0) ruby/1.9.3
```

I couldn't figure out how to get it to parse it as a string (tried `filename=...`, but no dice), so i changed it to be a positional argument instead.