Specify Unirest version

With the current Sendgrid package, installation fails with the following message without minimum stability set to `dev`:

```
Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - sendgrid/sendgrid 2.0.4 requires mashape/unirest-php dev-master -> no matching package found.
    - Installation request for sendgrid/sendgrid 2.0.4 -> satisfiable by sendgrid/sendgrid[2.0.4].

Potential causes:
 - A typo in the package name
 - The package is not available in a stable-enough version according to your minimum-stability setting
   see <https://groups.google.com/d/topic/composer-dev/_g3ASeIFlrc/discussion> for more details.

Read <http://getcomposer.org/doc/articles/troubleshooting.md> for further common problems.      
```

I've specified the most recent of two *releases* for Unirest with this PR.