Update mini_racer gem

Trying to bundle on macOS Mojave I would get the following error:

    ruby-2.5.0/ruby/intern.h:47:19: warning: 'register' storage class
    specifier is deprecated and incompatible with C++17
    [-Wdeprecated-register]
    void rb_mem_clear(register VALUE*, register long);
                      ^~~~~~~~~
    ruby-2.5.1/include/ruby-2.5.0/ruby/intern.h:47:36: warning:
    'register' storage class specifier is deprecated and incompatible
    with C++17 [-Wdeprecated-register]
    void rb_mem_clear(register VALUE*, register long);
                                       ^~~~~~~~~

This was fixed by bumping the gem from 0.2.0 to 0.2.4.

rails/Gemfile.lock

@@ -147,7 +147,7 @@ GEM
     mimemagic (0.3.2)
     mini_mime (1.0.1)
     mini_portile2 (2.3.0)
-    mini_racer (0.2.0)
+    mini_racer (0.2.4)
       libv8 (>= 6.3)
     minitest (5.11.3)
     momentjs-rails (2.20.1)
@@ -323,4 +323,4 @@ RUBY VERSION
    ruby 2.5.1p57
 
 BUNDLED WITH
-   1.16.5
+   1.17.1