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 see, I misinterpreted your question. This test fails without my code changes:

```console
  1) figaro heroku:set handles values that are of Fixnum class
     Failure/Error: run_simple("figaro heroku:set -e test")
       Exit status was 1 but expected it to be 0. Output:
       
       /Users/Home/ruby_projects/figaro/lib/figaro/cli/task.rb:28:in `system': no implicit conversion of Fixnum into String (TypeError)
       	from /Users/Home/ruby_projects/figaro/lib/figaro/cli/task.rb:28:in `block in system'
       	from /Users/Home/.rvm/gems/ruby-2.1.2@global/gems/bundler-1.6.2/lib/bundler.rb:235:in `block in with_clean_env'
       	from /Users/Home/.rvm/gems/ruby-2.1.2@global/gems/bundler-1.6.2/lib/bundler.rb:222:in `with_original_env'
       	from /Users/Home/.rvm/gems/ruby-2.1.2@global/gems/bundler-1.6.2/lib/bundler.rb:228:in `with_clean_env'
       	from /Users/Home/ruby_projects/figaro/lib/figaro/cli/task.rb:28:in `system'
       	from /Users/Home/ruby_projects/figaro/lib/figaro/cli/heroku_set.rb:7:in `run'
       	from /Users/Home/ruby_projects/figaro/lib/figaro/cli/task.rb:9:in `run'
       	from /Users/Home/ruby_projects/figaro/lib/figaro/cli.rb:36:in `block in <class:CLI>'
       	from /Users/Home/.rvm/gems/ruby-2.1.2/gems/thor-0.19.1/lib/thor/command.rb:27:in `run'
       	from /Users/Home/.rvm/gems/ruby-2.1.2/gems/thor-0.19.1/lib/thor/invocation.rb:126:in `invoke_command'
       	from /Users/Home/.rvm/gems/ruby-2.1.2/gems/thor-0.19.1/lib/thor.rb:359:in `dispatch'
       	from /Users/Home/.rvm/gems/ruby-2.1.2/gems/thor-0.19.1/lib/thor/base.rb:440:in `start'
       	from /Users/Home/ruby_projects/figaro/bin/figaro:5:in `<top (required)>'
       	from /Users/Home/.rvm/gems/ruby-2.1.2/bin/figaro:23:in `load'
       	from /Users/Home/.rvm/gems/ruby-2.1.2/bin/figaro:23:in `<main>'
       	from /Users/Home/.rvm/gems/ruby-2.1.2/bin/ruby_executable_hooks:15:in `eval'
       	from /Users/Home/.rvm/gems/ruby-2.1.2/bin/ruby_executable_hooks:15:in `<main>'
     # ./spec/figaro/cli/heroku_set_spec.rb:68:in `block (2 levels) in <top (required)>'
```