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)

I changed this behavior because the `var` and `vars` methods previously produced a string that would look like:

```console
heroku config:set app_id="app_id" api_key="api_key" secret="secret"
```
After my code changes the generated string will include the key's value, like this:

```console
heroku config:set app_id="2954" api_key="7381a978f7" secret="abdc3b89"
```

My code changes should not change the behavior of special characters in string values, but I did a test for some special characters. Dollar signs, double-quotes, and backticks would need to be escaped in the `application.yml` as in this test example:
```yml
TEST_CONFIG_VALUE_QUOTES: 1!2@3#4\$5%6^7&8*9(0)1-2_3=4+5'6'7'8\"9\"0\"1.2,3<4>5:6;7\`8~9[0]1{2}
```

If I looked further into this issue, I might be able to eliminate the need for escaping $, ", and `.