Update num to 0.3

[adumbidiot]

This PR updates num to 0.3. Previously, an old version of num-rational was pulled in through:

num-rational v0.2.4
     └── num v0.2.1
         └── conrod_core v0.70.0
             ├── conrod_piston v0.70.0
              ...

while a newer version of num-rational was pulled in through:

num-rational v0.3.0
     └── image v0.23.10
         ├── piston-gfx_texture v0.41.0
         │   └── piston2d-gfx_graphics v0.70.0
         │       └── piston_window v0.113.0
          ...

Updating num allows only one version of num-rational to be pulled in.