Added webrick gem to make the development docker container work

src/Gemfile

@@ -19,6 +19,10 @@ group :jekyll_plugins do
   gem "jekyll-coffeescript"
 end
 
+group :development do
+  gem "webrick"
+end
+
 # Windows and JRuby does not include zoneinfo files, so bundle the tzinfo-data gem
 # and associated library.
 platforms :mingw, :x64_mingw, :mswin, :jruby do
@@ -28,4 +32,3 @@ end
 
 # Performance-booster for watching directories on Windows
 gem "wdm", "~> 0.1.1", :platforms => [:mingw, :x64_mingw, :mswin]
-