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)

Actually, I don't remember what issue I was having in my note above.  Now, all tests are passing with my code changes. I think the problem was related to my Bundler settings.

```console
$ rspec
.....................................................................................

Finished in 16.06 seconds
85 examples, 0 failures

Randomized with seed 15819
```