image_tag plugin fix: provide alt attribute when title is available.

  String#gsub! return nil when no substitutions were performed.
Rubypants: avoid to "educate_dashes" HTML comments close tags.
Disable unneeded MathJax for now
Merge remote-tracking branch 'kAworu/fix-rubypants-html-comments-tag-break' into source
Merge remote-tracking branch 'kAworu/fix-image-tag-alt' into source