Bump mini_racer from 0.1.15 to 0.2.4

Bumps [mini_racer](https://github.com/discourse/mini_racer) from 0.1.15 to 0.2.4.
- [Release notes](https://github.com/discourse/mini_racer/releases)
- [Changelog](https://github.com/rubyjs/mini_racer/blob/master/CHANGELOG)
- [Commits](https://github.com/discourse/mini_racer/compare/v0.1.15...v0.2.4)

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

Gemfile

@@ -106,7 +106,7 @@ group :assets do
   gem 'coffee-rails', '~> 3.2.1'
   gem 'compass-rails'
 
-  gem 'mini_racer', '0.1.15'
+  gem 'mini_racer', '0.2.4'
   # Previously we found that libv8 6.7.288.46.1 breakis the compilation of mini_racer.
   # Now we see that we need to set the version explicitly. Nothing else depends on libv8.
   gem 'libv8', '6.3.292.48.1'

Gemfile.lock

@@ -475,8 +475,8 @@ GEM
     mime-types (1.25.1)
     mini_mime (1.0.1)
     mini_portile2 (2.1.0)
-    mini_racer (0.1.15)
-      libv8 (~> 6.3)
+    mini_racer (0.2.4)
+      libv8 (>= 6.3)
     momentjs-rails (2.20.1)
       railties (>= 3.1)
     money (5.1.1)
@@ -773,7 +773,7 @@ DEPENDENCIES
   knapsack
   letter_opener (>= 1.4.1)
   libv8 (= 6.3.292.48.1)
-  mini_racer (= 0.1.15)
+  mini_racer (= 0.2.4)
   momentjs-rails
   newrelic_rpm (~> 3.0)
   nokogiri (>= 1.6.7.1)