Specify Unirest version

With the current Sendgrid package, installation fails with the following message:

```
Your requirements could not be resolved to an installable set of packages.                             
                                                                                                       
  Problem 1                                                                                            
    - Installation request for sendgrid/sendgrid 2.0.4 -> satisfiable by sendgrid/sendgrid[2.0.4].     
    - sendgrid/sendgrid 2.0.4 requires mashape/unirest-php dev-master -> no matching package found.    
                                                                                                       
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.