Bump ffi from 1.9.18 to 1.11.1

Bumps [ffi](https://github.com/ffi/ffi) from 1.9.18 to 1.11.1.
- [Release notes](https://github.com/ffi/ffi/releases)
- [Changelog](https://github.com/ffi/ffi/blob/master/CHANGELOG.md)
- [Commits](ffi/ffi@1.9.18...1.11.1)

Gemfile.lock

@@ -33,7 +33,7 @@ GEM
     debug_inspector (0.0.3)
     diff-lcs (1.3)
     docile (1.1.5)
-    ffi (1.9.18)
+    ffi (1.11.1)
     gherkin (4.1.3)
     json (2.1.0)
     method_source (0.8.2)