Depend directly on Jekyll, instead of indirectly through github-pages (#92)

The github-pages gem is not maintained anymore, and depends on old versions of
its dependencies, which seem to be incompatible with Netlify's Ruby version.

And it looks like we don't use github-pages, anyway.

Author: progval

Gemfile

@@ -1,3 +1,4 @@
 source "https://rubygems.org"
 
-gem "github-pages", group: :jekyll_plugins
+gem "jekyll"
+gem "jekyll-sitemap"