Add GD extension to requirements

Received this during a composer install:

```
  Problem 1
    - intervention/image 1.4.5 requires ext-gd * -> the requested PHP extension gd is missing from your system.
    - intervention/image 1.4.4 requires ext-gd * -> the requested PHP extension gd is missing from your system.
    - intervention/image 1.4.3 requires ext-gd * -> the requested PHP extension gd is missing from your system.
    - intervention/image 1.4.2 requires ext-gd * -> the requested PHP extension gd is missing from your system.
    - intervention/image 1.4.1 requires ext-gd * -> the requested PHP extension gd is missing from your system.
    - wardrobe/core 1.1.x-dev requires intervention/image 1.4.* -> satisfiable by intervention/image[1.4.1, 1.4.2, 1.4.3, 1.4.4, 1.4.5].
    - Installation request for wardrobe/core 1.1.* -> satisfiable by wardrobe/core[1.1.x-dev].
```