fixed h3, h4 font-sizing for gitlab-flavored markdown (solves issue #6211)

[GitLab Markdown Documentation](https://github.com/gitlabhq/gitlabhq/blob/master/doc/markdown/markdown.md#h2) shows h2 & h3 as being different font-sizes, correctly; however, currently only the padding is different in the sass mixin. This issue was opened by @rskuipers in issue [#6211](https://github.com/gitlabhq/gitlabhq/issues/6211).

The README displays the proper font sizes, which should be:
h3 {font-size: 1.5em}
h4 {font-size: 1.2em}