Update favicon emoji length limit (alshedivat#1099)

Since, a image file will always contain a dot and a three letter
extension at least (ex: `.jpg`, `.png`, `.jpeg`, etc). Therefore we can
safely set the limit up to 4.

fixes alshedivat#1096

_includes/head.html

@@ -22,7 +22,7 @@
     <link rel="stylesheet" href="https://cdn.jsdelivr.net/gh/jwarby/jekyll-pygments-themes@master/{{ site.highlight_theme_light | append: '.css' }}" media="" id="highlight_theme_light" />
 
     <!-- Styles -->
-    {% if site.icon.size < 3 %}
+    {% if site.icon.size <= 4 %}
     <link rel="shortcut icon" href="data:image/svg+xml,<svg xmlns=%22http://www.w3.org/2000/svg%22 viewBox=%220 0 100 100%22><text y=%22.9em%22 font-size=%2290%22>{{ site.icon }}</text></svg>">
     {% elsif site.icon != blank %}
     <link rel="shortcut icon" href="{{ site.icon | prepend: '/assets/img/' | relative_url}}"/>