Update ffi gem to v1.15.3

We're shipping three different versions of this gem in Omnibus.  Update
to the latest to avoid wasting space.

Diff: https://my.diffend.io/gems/ffi/1.15.1/1.15.3

Gemfile.lock

@@ -381,7 +381,7 @@ GEM
     fast_blank (1.0.0)
     fast_gettext (1.6.0)
     ffaker (2.10.0)
-    ffi (1.15.1)
+    ffi (1.15.3)
     ffi-compiler (1.0.1)
       ffi (>= 1.0.0)
       rake