Pass key and value to vars method

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)