BUG: Support local generation of the site using github/pages-gem

As described in the Gemfile, if "github-pages" gem is used, the "jekyll"
one should be commented out. Failure to do so leads to the generation
of an incomplete site unable to reference its style-sheet.

References:
* https://github.com/github/pages-gem
* Starefossen/docker-github-pages#74 (comment)

Gemfile

@@ -8,7 +8,7 @@ source "https://rubygems.org"
 #
 # This will help ensure the proper Jekyll version is running.
 # Happy Jekylling!
-gem "jekyll", "~> 3.9"
+# gem "jekyll", "~> 3.9"
 
 # This is the default theme for new Jekyll sites. You may change this to anything you like.
 gem "minima", "~> 2.0"

Gemfile.lock

@@ -276,7 +276,6 @@ PLATFORMS
 
 DEPENDENCIES
   github-pages (~> 219)
-  jekyll (~> 3.9)
   jekyll-feed (~> 0.6)
   minima (~> 2.0)
   tzinfo-data