Bump ffi from 1.9.18 to 1.11.1 (#955)

Author: dependabot[bot]

Gemfile.lock

@@ -14,7 +14,7 @@ GEM
       rubyzip (~> 1.0)
     domain_name (0.5.20170404)
       unf (>= 0.0.5, < 1.0.0)
-    ffi (1.9.18)
+    ffi (1.11.1)
     forwardable-extended (2.6.0)
     gli (2.17.1)
     http-cookie (1.0.3)