Removed github-markdown dependency from gollum-lib.gemspec

Explicit dependency on 'github-markdown' in gemspec makes it difficult to use another Markdown engine (i.e. kramdown), because 'github-markdown' gem checks for 'github-markdown' and uses it by default if present.

When this dependency is removed, the user need not touch gollum-lib configuration at all, just include his/her favourite Markdown engine gem, and it is used transparently by 'github-markdown' upstream.