update mini_racer gem to fix libv8 error (#466)

Author: hussainakram

Gemfile.lock

@@ -125,7 +125,7 @@ GEM
     json (2.2.0)
     launchy (2.4.3)
       addressable (~> 2.3)
-    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)
@@ -141,8 +141,8 @@ GEM
     mimemagic (0.3.3)
     mini_mime (1.0.1)
     mini_portile2 (2.4.0)
-    mini_racer (0.2.4)
-      libv8 (>= 6.3)
+    mini_racer (0.2.14)
+      libv8 (> 7.3)
     minitest (5.11.3)
     multi_json (1.13.1)
     nio4r (2.3.1)