Update

Gemfile.lock

@@ -16,7 +16,7 @@ GEM
       citeproc (>= 1.0.2, < 2.0)
       csl (~> 1.4)
     coderay (1.1.1)
-    colorator (0.1)
+    colorator (1.1.0)
     colored (1.2)
     csl (1.4.4)
       namae (~> 0.7)
@@ -25,6 +25,7 @@ GEM
     ethon (0.9.0)
       ffi (>= 1.3.0)
     ffi (1.9.14)
+    forwardable-extended (2.6.0)
     html-proofer (3.0.6)
       activesupport (~> 4.2)
       addressable (~> 2.3)
@@ -35,13 +36,14 @@ GEM
       typhoeus (~> 0.7)
       yell (~> 2.0)
     i18n (0.7.0)
-    jekyll (3.1.6)
-      colorator (~> 0.1)
+    jekyll (3.2.1)
+      colorator (~> 1.0)
       jekyll-sass-converter (~> 1.0)
       jekyll-watch (~> 1.1)
       kramdown (~> 1.3)
       liquid (~> 3.0)
       mercenary (~> 0.3.3)
+      pathutil (~> 0.9)
       rouge (~> 1.7)
       safe_yaml (~> 1.0)
     jekyll-sass-converter (1.4.0)
@@ -51,7 +53,7 @@ GEM
       citeproc-ruby (~> 1.0)
       csl-styles (~> 1.0)
       jekyll (~> 3.0)
-    jekyll-watch (1.4.0)
+    jekyll-watch (1.5.0)
       listen (~> 3.0, < 3.1)
     json (1.8.3)
     kramdown (1.11.1)
@@ -70,6 +72,8 @@ GEM
       mini_portile2 (~> 2.1.0)
       pkg-config (~> 1.1.7)
     parallel (1.9.0)
+    pathutil (0.14.0)
+      forwardable-extended (~> 2.6)
     pkg-config (1.1.7)
     pry (0.10.4)
       coderay (~> 1.1.0)

_plugins/extra-liquid-filters.rb

-module Jekyll
-  module ExtraFilters
- 
-    # Filters input against an arbitrary expression, where each item is assigned to variable
-    def where_exp(input, variable, expression)
-      c = parse_comparison(expression)
-      @context.stack do
-        input.select do |object|
-          @context[variable] = object
-          c.evaluate(@context)
-        end
-      end
-    end
-
-    private
-
-    def parse_comparison(exp)
-      p = Liquid::Parser.new(exp)
-      a = p.expression
-      if op = p.consume?(:comparison)
-        b = p.expression
-        c = Liquid::Condition.new(a, op, b)
-      else
-        c = Liquid::Condition.new(a)
-      end
-      p.consume(:end_of_string)
-
-      c
-    end
-  end
-end
-
-Liquid::Template.register_filter(Jekyll::ExtraFilters)