get bundle install working again

it was telling me:

>Downloading html-proofer-3.8.0 revealed dependencies not in the API or the lockfile (nokogiri (~> 1.8.1)).
>Either installing with `--full-index` or running `bundle update html-proofer` should fix the problem.

and Travis-CI was saying the same thing at e.g.
https://travis-ci.org/scala/scala-lang/builds/402903201

so I did `bundle update html-proofer` and these are the resulting
changes.

Author: SethTisue

Gemfile.lock

@@ -1,9 +1,9 @@
 GEM
   remote: https://rubygems.org/
   specs:
-    activesupport (5.1.4)
+    activesupport (5.2.0)
       concurrent-ruby (~> 1.0, >= 1.0.2)
-      i18n (~> 0.7)
+      i18n (>= 0.7, < 2)
       minitest (~> 5.1)
       tzinfo (~> 1.1)
     addressable (2.5.2)
@@ -13,18 +13,18 @@ GEM
     concurrent-ruby (1.0.5)
     ethon (0.11.0)
       ffi (>= 1.3.0)
-    ffi (1.9.18)
+    ffi (1.9.25)
     forwardable-extended (2.6.0)
-    html-proofer (3.8.0)
+    html-proofer (3.9.1)
       activesupport (>= 4.2, < 6.0)
       addressable (~> 2.3)
       colorize (~> 0.8)
       mercenary (~> 0.3.2)
-      nokogiri (~> 1.8.2)
+      nokogiri (~> 1.8.1)
       parallel (~> 1.3)
       typhoeus (~> 1.3)
       yell (~> 2.0)
-    i18n (0.9.3)
+    i18n (1.0.1)
       concurrent-ruby (~> 1.0)
     jekyll (3.5.2)
       addressable (~> 2.4)
@@ -50,13 +50,13 @@ GEM
       rb-inotify (~> 0.9, >= 0.9.7)
     mercenary (0.3.6)
     mini_portile2 (2.3.0)
-    minitest (5.11.2)
-    nokogiri (1.8.1)
+    minitest (5.11.3)
+    nokogiri (1.8.4)
       mini_portile2 (~> 2.3.0)
     parallel (1.12.1)
     pathutil (0.14.0)
       forwardable-extended (~> 2.6)
-    public_suffix (3.0.0)
+    public_suffix (3.0.2)
     rack (1.6.5)
     rack-jekyll (0.5.0)
       jekyll (>= 1.3)
@@ -76,7 +76,7 @@ GEM
     thread_safe (0.3.6)
     typhoeus (1.3.0)
       ethon (>= 0.9.0)
-    tzinfo (1.2.4)
+    tzinfo (1.2.5)
       thread_safe (~> 0.1)
     yell (2.0.7)