Markdown typography CSS fixes

The `!important` flags recently added to the `md-typography`
mixin's `pre` elements were overriding the background and
foreground colors for syntax highlighting.

| Before | After |
|:------:|:-----:|
| ![Screen_Shot_2015-09-25_at_3.32.32_PM](https://gitlab.com/gitlab-org/gitlab-ce/uploads/802163bd9f46864ac89e7be165951f07/Screen_Shot_2015-09-25_at_3.32.32_PM.png) | ![Screen_Shot_2015-09-25_at_3.33.30_PM](https://gitlab.com/gitlab-org/gitlab-ce/uploads/6be72036653378405320085324dff5ad/Screen_Shot_2015-09-25_at_3.33.30_PM.png) |

See merge request !1442