Update minima to 2.1.1

Gemfile

@@ -1,4 +1,2 @@
 source 'https://rubygems.org'
 gem 'github-pages', group: :jekyll_plugins
-
-gem "minima", '2.0.0'

_sass/minima.scss

@@ -1,3 +1,5 @@
+@charset "utf-8";
+
 // This file is the same as the one provided by the the minima theme (ie. unmodified),
 // but must be reproduced here because of our custom syntax-highlighting partial
 
@@ -38,9 +40,13 @@ $on-laptop:        800px !default;
   }
 }
 
+@mixin relative-font-size($ratio) {
+  font-size: $base-font-size * $ratio;
+}
+
 // Import partials.
 @import
   "minima/base",
   "minima/layout",
   "minima/syntax-highlighting"
-;
+;