change json frontmatter delimeter to the ironic ;;; to avoid interfering with mustache {{{

middleman-core/fixtures/frontmatter-app/source/json-front-matter-2.php.erb

@@ -1,7 +1,7 @@
-{{{
+;;;
   "layout": false,
   "title":  "This is the title"
-}}}
+;;;
 
 <h1><%= data.page.title %></h1>
 <?php echo "sup"; ?>

middleman-core/fixtures/frontmatter-app/source/json-front-matter-auto.erb

@@ -1,6 +1,6 @@
-{{{
+;;;
   "layout": false,
   "title":  "This is the title"
-}}}
+;;;
 
 <h1><%= data.page.title %></h1>

middleman-core/fixtures/frontmatter-app/source/json-front-matter.html.erb

@@ -1,6 +1,6 @@
-{{{
+;;;
   "layout": false,
   "title":  "This is the title"
-}}}
+;;;
 
 <h1><%= data.page.title %></h1>

middleman-core/lib/middleman-core/core_extensions/front_matter.rb

@@ -104,7 +104,7 @@ module Middleman::CoreExtensions
           content = content.sub(json_regex, "")
 
           begin
-            json = ($1+$2).sub("{{{", "{").sub("}}}", "}")
+            json = ($1+$2).sub(";;;", "{").sub(";;;", "}")
             data = ActiveSupport::JSON.decode(json)
           rescue => e
             puts "JSON Exception: #{e.message}"