Bump mini_racer from 0.2.6 to 0.2.8

Bumps [mini_racer](https://github.com/discourse/mini_racer) from 0.2.6 to 0.2.8.
- [Release notes](https://github.com/discourse/mini_racer/releases)
- [Changelog](https://github.com/discourse/mini_racer/blob/master/CHANGELOG)
- [Commits](rubyjs/mini_racer@v0.2.6...v0.2.8)

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

Gemfile.lock

@@ -149,7 +149,7 @@ GEM
     mini_magick (4.9.5)
     mini_mime (1.0.2)
     mini_portile2 (2.4.0)
-    mini_racer (0.2.6)
+    mini_racer (0.2.8)
       libv8 (>= 6.9.411)
     minitest (5.13.0)
     minitest-reporters (1.3.8)