DEV: update mini racer version

Mini Racer 0.2.5 provides support for libv8 7.3 (so we just upgraded from
Chrome 67 -> 73 JS engine wise)

Gemfile.lock

@@ -175,7 +175,7 @@ GEM
     jwt (2.1.0)
     kgio (2.11.2)
     kramdown (1.17.0)
-    libv8 (6.7.288.46.1)
+    libv8 (7.3.492.27.1)
     listen (3.1.5)
       rb-fsevent (~> 0.9, >= 0.9.4)
       rb-inotify (~> 0.9, >= 0.9.7)
@@ -203,8 +203,8 @@ GEM
     method_source (0.8.2)
     mini_mime (1.0.1)
     mini_portile2 (2.4.0)
-    mini_racer (0.2.4)
-      libv8 (>= 6.3)
+    mini_racer (0.2.5)
+      libv8 (>= 6.9.411)
     mini_scheduler (0.9.2)
       sidekiq
     mini_sql (0.2.1)