Update to latest version of FFI to resolve security alert

Author: githubteacher

Gemfile.lock

@@ -19,7 +19,7 @@ GEM
     execjs (2.7.0)
     faraday (0.12.2)
       multipart-post (>= 1.2, < 3)
-    ffi (1.9.18)
+    ffi (1.9.24)
     forwardable-extended (2.6.0)
     gemoji (3.0.0)
     github-pages (148)