Adding background colour fallback for page-hero

Whilst the background image is loading, a solid background colour will be visible:

![screen shot 2014-10-20 at 14 10 11](https://cloud.githubusercontent.com/assets/883598/4701597/b47c8bfa-585a-11e4-9bc6-31e221925236.png)

This isn't currently the case where `background` of `.page-hero` is set separately as a solid colour and `background` of `.page-hero--gyms` (for example) is set as an image.

The `.page-hero` background colour is ignored completely when the rules are combined (resulting in a white background before the image has loaded), so each individual hero eg `.page-hero--gyms` needs the background colour to be set together with the image.