Update gems

I updated some of these gems for a different project, and
suddenly `rake build` was giving very different output
compared to the current gh-pages. This commit makes sure
everyone updating gh-pages is using reasonably similar
versions, so we avoid spurious diffs.

docs/Gemfile

@@ -4,3 +4,4 @@ gem "rake"
 gem "bonsai"
 gem "maruku"
 gem "ronn"
+gem "mustache", "<1.0" # to support ruby <2.0