Merge pull request #22 from Shopify/upgrade_ffi

ffi: bump to latest

Author: mistydemeo

Gemfile.lock

@@ -9,7 +9,7 @@ GEM
       ffi (~> 1.0, >= 1.0.11)
     clamp (1.0.1)
     dotenv (2.5.0)
-    ffi (1.9.25)
+    ffi (1.15.4)
     fpm (1.10.2)
       arr-pm (~> 0.0.10)
       backports (>= 2.6.2)