Update ffi gem to 1.11.1

This reduces duplication on Omnibus GitLab and saves about ~8 MB of disk
space.

Full list of changes:
https://github.com/ffi/ffi/blob/master/CHANGELOG.md

Gemfile.lock

@@ -236,7 +236,7 @@ GEM
     fast_blank (1.0.0)
     fast_gettext (1.6.0)
     ffaker (2.10.0)
-    ffi (1.10.0)
+    ffi (1.11.1)
     flipper (0.13.0)
     flipper-active_record (0.13.0)
       activerecord (>= 3.2, < 6)