Manually update rouge to 1.9.1

Since gollum-lib blocks rouge from updating, let's bump the version
manually. The fix has already been accepted upstream:
https://github.com/gollum/gollum-lib/commit/9c042ea0914a7714abbda9d89e234c8a79fba893

Gemfile.lock

@@ -288,7 +288,7 @@ GEM
       github-markup (~> 1.3.1)
       gollum-grit_adapter (~> 0.1, >= 0.1.1)
       nokogiri (~> 1.6.4)
-      rouge (~> 1.7.4)
+      rouge (~> 1.9)
       sanitize (~> 2.1.0)
       stringex (~> 2.5.1)
     gon (5.0.1)
@@ -536,7 +536,7 @@ GEM
       netrc (~> 0.7)
     rinku (1.7.3)
     rotp (1.6.1)
-    rouge (1.7.7)
+    rouge (1.9.1)
     rqrcode (0.4.2)
     rqrcode-rails3 (0.1.7)
       rqrcode (>= 0.4.2)