Upgrade heroku to fix an issue with RestClient gem

Before upgrading, I would get these errors:

```
 !    Heroku client internal error.
 !    Search for help at: https://help.heroku.com
 !    Or report a bug at: https://github.com/heroku/heroku/issues/new

    Error:       uninitialized constant Heroku::Command::RestClient (NameError)
    Backtrace:   /opt/boxen/heroku/lib/heroku/command.rb:225:in `rescue in run'
                 /opt/boxen/heroku/lib/heroku/command.rb:269:in `run'
                 /opt/boxen/heroku/lib/heroku/cli.rb:28:in `start'
                 /opt/boxen/heroku/bin/heroku:24:in `<main>'

    Command:     heroku releases
    Version:     heroku-toolbelt/3.6.0 (x86_64-darwin13.0) ruby/2.1.1
```