Fix figaro config:set CLI 

This fixes the CLI command: `$ figaro heroku:set`

I tested this on my Mac OS X (Mavericks) terminal. I'm unable to test the Windows string format, but if the % symbols are necessary at the Windows command line, then this fix should work. 

This possibly fixes Issues #127 and #139

If someone wants to help with writing a tests for this, let me know.

![mib1](https://cloud.githubusercontent.com/assets/4951910/3491900/7332c2f8-059f-11e4-8f7a-973095e18710.gif)

That's not quite accurate. Previously, the string that was produced was either:

```
heroku config:set app_id="$app_id" api_key="$api_key" secret="$secret"
```

(note the dollar signs), or this on Windows:

```
heroku config:set app_id="%app_id%" api_key="%api_key%" secret="%secret%"
```

This was done to avoid any/all special character escaping issues. The first argument given to the `system` method (if a hash) is the ENV variables under which to run the command. See `system` [docs](http://www.ruby-doc.org/core-2.1.2/Kernel.html#method-i-system) for more information, specifically the bit about ENV variables under the `spawn` [docs](http://www.ruby-doc.org/core-2.1.2/Kernel.html#method-i-spawn).