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

app/assets/stylesheets/framework/typography.scss

@@ -101,9 +101,9 @@
pre {
margin: 12px 0 12px 0 !important;
-    background-color: #f8fafc !important;
+    background-color: #f8fafc;
font-size: 13px !important;
-    color: #5b6169 !important;
+    color: #5b6169;
line-height: 1.6em !important;
@include border-radius(2px);
}