update html-proofer gem

This fixes a [security issue with nokogiri v1.7.1](sparklemotion/nokogiri#1634)

Gemfile

@@ -13,4 +13,4 @@ gem 'liquid', '~> 3.0.x'
 
 # Plugins
 gem 'jekyll-sitemap', '1.0.0'
-gem 'html-proofer', '3.6.0'
+gem 'html-proofer', "~> 3.7.x"

Gemfile.lock

@@ -1,7 +1,7 @@
 GEM
   remote: https://rubygems.org/
   specs:
-    activesupport (5.0.2)
+    activesupport (5.1.1)
       concurrent-ruby (~> 1.0, >= 1.0.2)
       i18n (~> 0.7)
       minitest (~> 5.1)
@@ -20,12 +20,12 @@ GEM
     execjs (2.6.0)
     ffi (1.9.10)
     forwardable-extended (2.6.0)
-    html-proofer (3.6.0)
+    html-proofer (3.7.2)
       activesupport (>= 4.2, < 6.0)
       addressable (~> 2.3)
       colored (~> 1.2)
       mercenary (~> 0.3.2)
-      nokogiri (~> 1.5)
+      nokogiri (~> 1.7)
       parallel (~> 1.3)
       typhoeus (~> 0.7)
       yell (~> 2.0)
@@ -56,10 +56,10 @@ GEM
       rb-inotify (~> 0.9, >= 0.9.7)
     mercenary (0.3.6)
     mini_portile2 (2.1.0)
-    minitest (5.10.1)
-    nokogiri (1.7.1)
+    minitest (5.10.2)
+    nokogiri (1.7.2)
       mini_portile2 (~> 2.1.0)
-    parallel (1.11.1)
+    parallel (1.11.2)
     pathutil (0.14.0)
       forwardable-extended (~> 2.6)
     public_suffix (2.0.5)
@@ -80,7 +80,7 @@ PLATFORMS
   ruby
 
 DEPENDENCIES
-  html-proofer (= 3.6.0)
+  html-proofer (~> 3.7.x)
   jekyll (~> 3.4.x)
   jekyll-coffeescript (~> 1.0.x)
   jekyll-sass-converter (~> 1.5.x)