Fix button shadow colour on product page
The button was inheriting a green box shadow from the default GOV.UK button style, which is typically used on a white background. On the blue background, a darker blue shadow looks better I think.
The 15% darkening is the same as used by the default green button: https://github.com/alphagov/govuk_frontend_toolkit/blob/a3fe44e00ef3e364376a138fbffa8495af9906df/stylesheets/design-patterns/_buttons.scss#L36
Before | After
![image](https://cloud.githubusercontent.com/assets/355079/21259760/ca7aa850-c37a-11e6-8f62-8c20d8043270.png) | ![image](https://cloud.githubusercontent.com/assets/355079/21259744/ba617322-c37a-11e6-8260-61fba018e7cf.png)
@nickcolley I have no idea now you spotted this, but thanks.