build(deps): bump mini_racer from 0.2.15 to 0.3.1 in /docs/v3

Bumps [mini_racer](https://github.com/discourse/mini_racer) from 0.2.15 to 0.3.1.
- [Release notes](https://github.com/discourse/mini_racer/releases)
- [Changelog](https://github.com/rubyjs/mini_racer/blob/master/CHANGELOG)
- [Commits](rubyjs/mini_racer@v0.2.15...v0.3.1)

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

docs/v3/Gemfile.lock

@@ -48,7 +48,7 @@ GEM
     i18n (0.7.0)
     json (2.3.0)
     kramdown (1.17.0)
-    libv8 (7.3.492.27.1)
+    libv8 (8.4.255.0)
     listen (3.0.8)
       rb-fsevent (~> 0.9, >= 0.9.4)
       rb-inotify (~> 0.9, >= 0.9.7)
@@ -96,8 +96,8 @@ GEM
       mime-types-data (~> 3.2015)
     mime-types-data (3.2019.1009)
     mini_portile2 (2.4.0)
-    mini_racer (0.2.15)
-      libv8 (> 7.3)
+    mini_racer (0.3.1)
+      libv8 (~> 8.4.255)
     minitest (5.14.0)
     multi_json (1.14.1)
     nokogiri (1.10.9)