Update gitlab-markup back to 1.5.1 in Gemfile

Signed-off-by: Rémy Coutable <remy@rymai.me>

Gemfile

@@ -101,7 +101,7 @@ gem 'seed-fu', '~> 2.3.5'
 # Markdown and HTML processing
 gem 'html-pipeline',        '~> 1.11.0'
 gem 'deckar01-task_list',   '1.0.6', require: 'task_list/railtie'
-gem 'gitlab-markup',        '~> 1.5.0'
+gem 'gitlab-markup',        '~> 1.5.1'
 gem 'redcarpet',            '~> 3.3.3'
 gem 'RedCloth',             '~> 4.3.2'
 gem 'rdoc',                 '~> 4.2'